Feats, one of those things that every character gets, aren't quite the same. Even if you carefully sidestep the lousy "+1 to something" ones, they're almost always set in what they can or can't do. Power Attack is a great feat, but it's not like there's a wide variety of things you can do with it. Surely you can get a versatile and open-ended feat you could use in a different manner every day? For just any character to pull that off, well, it will take some real... trickery, won't it?
Once per day as a standard action, you can create an exact duplicate of yourself up to 30 feet away. You can control this simulacrum's movements as a free action. The image becomes more "real" as you advance in level. This ability is usable up to a maximum of 1 minute per level each day. The simulacrum combines the characteristics of the silent image (PH 279) and unseen servant (PH 297) spells. Its hit points are equal to 6 + your character level. It ignores terrain effects and moves like an unseen servant , making no noise in the process, though it makes normal motions while traveling. If you have a fly speed, the image appears to fly when not in contact with the ground; otherwise, it walks on open air. You can do anything with this image that you could do with the unseen servant spell and are under the same limitations.
When you attain 5th level, you gain more control over the simulacrum. At this point, it behaves more like a major image spell (PH 252), though it still performs actions like an unseen servant . As long as the simulacrum remains within 5 feet of your position (and you can direct it to do so as a free action), you can perform a Bluff check in combat as a swift action. You gain a +4 bonus on this check, which is opposed by your opponents' Sense Motive checks. If you are successful, your foes believe the image is you (and vice versa) for 1 round.
Once you reach 10th level, you can spend a swift action to transfer your perceptions to the simulacrum and perceive the world from its point of view rather than your own. You can move the image as if it were your own body, using the simulacrum's movement characteristics (ignoring terrain, "flying," and the like). You can also perform any skill and ability checks using the simulacrum, as long as they do not require a Strength score higher than 2. The image has phantom versions of all your equipment (such as lockpicks), but any such items that lose direct contact with it immediately dissipate. Phantom versions of your equipment do not have any of the magical or alchemical effects that your normal equipment has.
When you attain 15th level, the image can become more "real" at your command. It gains a Strength score equal to one-half your own and can perform combat actions using phantom versions of your gear. Any of these items that lose contact with the image dissipate immediately. Thus, the image can wield a sword, but if it shoots a bow, the arrow disappears as soon as it is fired. Equipment that extends more than 5 feet from the image (such as a length of rope) also disappears beyond that distance. In addition, you can now cast spells originating from the image as if with a project image spell (PH 265), with one exception. If you lose line of effect to the image, the effect does not end; you just can't cast spells originating from the image until you gain line of effect again.
You can select this feat multiple times, gaining one additional daily use each time you take it. If you have the ability to turn or rebuke undead, you gain one additional daily use of this feat for each three daily turn or rebuke uses you expend.
Trickery Devotion, one of the Devotion feats from Complete Champion, is essentially unseen servant+. It's one of the feats that rewards creativity the absolute most, especially with how well it scales as you level up. It has no prerequisites, meaning even a level 1 commoner can inexplicably make a nonmagical duplicate of himself, but it's better for clerics or other characters with a turning pool. After all, being able to use it only once per day is pretty lame, but we work with what we've got in the name of squeezing usefulness out of your feat slots.
As a quick aside, I think the wording on the feat here incidentally puts to rest the debate over whether or not an unseen servant can move through the air. While the spell itself doesn't definitively say one way or the other, Trickery Devotion notes that your simulacrum "ignores terrain effects and moves like an unseen servant," and also "the image appears to fly when not in contact with the ground; otherwise, it walks on open air." Thus, if it moves just like an unseen servant and can move through the air freely (how it appears while it does so is incidental) then we are forced to conclude that the unseen servant can also move through the air freely. Tidy, isn't it?
Anyways, the question we're inevitably going to approach: What exactly can you do with Trickery Devotion? The idea of a duplicate is certainly an intriguing one, even with restrictions in place, and by 15th level it becomes 'real' enough to take combat actions. At this point it's more like a mini-astral projection, with many of the same abuses being in place. Anything that happens to the clone does not happen to you unless you are in the effect's area, for instance. The feat is poorly balanced, to put it simply, but the game designer's bane is our boon.
So at first, your duplicate is a glorified unseen servant, except it's more like a seen servant. Fortunately, all of the same abuses that can be done with a servant can be performed with your duplicate, although the text suggests it's locked in at having to stay within 30 feet. Of course, as nice as unseen servant is, having one use of it per day isn't exactly worth a feat slot, especially as your get up in level and bamboozling your foes with Kevin McCallister-style traps starts to lose its lustre. Starting at 5th level, then, you can make a Bluff check to fool your foes into thinking that the duplicate is in fact you. This can be a pretty big deal, considering that should essentially negate all attacks and targeted spells against you for the round. The feat description still ensures us that it's functionally mostly like unseen servant, and as dictated by unseen servant, "it cannot be killed, but it dissipates if it takes 6 points of damage from area attacks." This strongly suggests to me that targeted attacks won't work: unless your opponent was sending a fireball at your feet anyway, no amount of sword blows or splinterbolts will harm your duplicate. Pretty handy for the skillful character, meaning a Charismatic rogue or a bard with glibness can be extremely difficult to take out.
At this point, your duplicate becomes more like project image, though without the ability to cast spells through it. You can see through its eyes and generally control its actions in a first-person view, for those of you who want to play Elder Scrolls for a minute per level every day. At this point it can perform any skills checks you can (though it still has Str 2,) no longer being stuck with the untrained-only DC 10 skill checks of an unseen servant. Having a decoy able to perform highly-skilled work a safe distance away can come in handy, especially since it can just fly around effortlessly in a way you might not be able to. Remember, however, that the duplicate still can't flank or threaten area, let alone attack, even if you're acting in its body.
What's really intriguing is that it also has 'phantom versions of all your equipment,' meaning we get access to astral projection abuse lite. For those of you not in the know, this basically means that since the duplicate has a copy of all of your items, you can use its copy of your disposable items without actually losing anything. Errata has added the very imperative and very understandable addendum that it doesn't have any magical or alchemical items, meaning no access to free scrolls, wand/staff charges or one-use items you already have. The call-out for alchemical items kind of surprises me, seeing as things like this almost always have an embargo on magic items only, leaving alchemy with its time to shine. As it happens, however, poisons are neither magical nor alchemical, so you do indeed get a copy of all your poisons every day. Unfortunately, any of your duplicate's items that lose physical contact with it disappear, so you might have to wait a few levels before you can make good on that poison.
At this point we're in the land of milk and honey as far as Trickery Devotion is concerned. You can cast spells through it like the more impressive feature of project image, making it an all-in-one package to fool your foes and direct attacks towards the more-resilient duplicate. Additionally, its Strength improves to one-half yours (probably still fairly low unless you're a Firbolg War Hulk) and it can now perform combat actions... whatever that means.
So once again, what exactly "combat actions" entails is poorly defined. Attacks (even full attacks,) tripping, disarming, all those are the obvious ones on the table. Personally, the next thing that comes to mind is Tome of Battle maneuvers. Whether they qualify or not isn't guaranteed, but considering I could imagine someone referring to maneuvers as 'combat actions' in casual conversation, it is tempting. This one probably comes down to your DM.
Let's say they do qualify. Since they aren't spells or anything of the sort, once you hit level 15, your duplicate will have an identical set of readied maneuvers that can be expended independent of your original character. Doubling your maneuver output is nasty enough, considering there are a number of maneuvers that don't require a high Str score to put the hurt on, but you and your duplicate can also trade White Raven Tactics back and forth. Even just having it target you with WRT and then refreshing the maneuver again and again should be an insurmountable advantage in most fights. Don't forget that it will have access to a stance, too, which can help out the whole party if carefully chosen - Tactics of the Wolf, Island of Blades and Leading the Charge are all helpful candidates.
Now, I'm willing to bet your duplicate will have access to all of your feats. Since it can use weapons and wear armour, then it's probably meant to have the same proficiencies as you, and profiencies are feats. Likewise, it can perform those mysterious "combat actions," which things like Improved Trip and Improved Sunder modify. Ergo, it likely has all the same feats you do.
Since it will probably have pretty low Strength, if you do want it to get in the thick of things, it's better suited to Weapon Finesse builds (especially if you add in Shadow Blade or Sneak Attack for the damage side of things.) A 15th level rogue with Craven and Tactics of the Wolf need never worry about being without a flanking partner if he can throw a duplicate of himself on the other side of a foe.
For feats with a limited number of uses, things get a little hairier. I'd posit that your duplicate can use these feats, but said uses draw from your pool of daily uses. For instance, if you used Travel Devotion and then your duplicate used Travel Devotion as well, you'd have to spend two turning attempts. After all, you still use your own spell slots when you cast spells "through" the simulacrum - same concept. If you can get your DM to rule otherwise, then more power to ya.
Psionic users of Trickery Devotion, regardless of magic-psionics transparency, likely can manifest through the duplicate but use their own power points - again, same deal as casting a spell. That said, if psionics and magic are completely segregated in your campaign, then please note with gleeful cackles that a psionic item is neither magic nor alchemical, and thus your duplicate will get fully-functional copies of all psionic items on you.
Anyone lucky enough to be playing a Fiend of Possession could use Trickery Devotion to create a duplicate body as a stand-in for themselves, which could then carry an item possessed by the actual character. Fiend of Possession looks like one of the most interesting characters to play, but it's a real pain for most player characters to qualify for.
Can your duplicate use auras if you're a dragon shaman or marshal? Could a duplicated marshal grant move actions? If you're a binder, can it bind a vestige? Can it use soulmelds and have its own essentia pool? If a duplicate draws the Donjon from a Deck of Many Things, is it imprisoned in your place? I really don't know where you draw the line. I'd want to say 'yes' to all of these, since A) Rewarding creativity is appropriate, B) It's still just a minute per day and C) None of these help casters, so think of it as a balancing factor.
If you're a maniac much like myself, then this last point may have given you a very dastardly idea indeed: making your duplicate use Trickery Devotion, giving you a duplicate of a duplicate. You'd have to order your duplicate to give its duplicate orders, and the duplicate once-removed would end up with a quarter of your Strength score... honestly, even if your DM lets your duplicate use feats, just don't do this. It really amazes me how, in a book so late in 3.5's lifespan as Complete Champion, they went ahead and printed an ability with the word "simulacrum" in it. Hadn't they read the letters? Hadn't they heard the screams? Haven't enough DMs had to bury their dearly departed campaign under an onslaught of mini-wizards? We here at 100 Points of Dawizard make a habit of exploring character options with terribly incomplete descriptions, though, and thus we must stare the most ridiculous implications square in the eye. Just be glad it's for a minute per day and available to any old character.