As we all know, classes that can cast spells will always be stronger than those that cannot. There's no quick fix for this disparity - every single spell printed is another option for the casters, yet sword-swingers have to pick from a variety of band-aid-type patches to make them more viable. So what if, instead, they had a way to cast spells of their own?
All right, yes, there is some rather lengthy fine print to that statement, but there is an efficient way for anyone to sling a spell every now and then. All it takes to qualify is the ability to swing a stick around - the harder, the better. Enter the spell-storing weapon special ability from the Dungeon Master's Guide: essentially it allows a spellcaster to store any one targeted spell of up to 3rd level in a melee weapon. Whenever the spell-storing weapon hits a creature, it can immediately cast the stored spell on the foe, regardless of who or what its wielder is. It can only store one spell at a time, but it can be refilled any number of times. Now, chances are you aren't going to take the time to store a spell in a weapon mid-combat, so it amounts to one quickened spell on the first strike in a fight. If the spell was stored the day before, it even amounts to one extra spell slot. Not bad for a +1 bonus!
So which spells are the best choices for your rider effect? The restriction to targeted spells means no Rays are permitted, so things like ray of exhaustion and enervation are off the table. The level cap of 3rd means you won't be wishing upon any stars - or anything else, for that matter. Still, there are a healthy number of effects that most assailants won't want cast on them within that criteria, some particular standouts being listed here.
Bands of Steel: The foe is immobilized if they fail a save, and entangled if they succeed - this is particularly good against foes who use wings to fly, and the spell is overall more versatile than earthbind. Most interestingly, this spell targets the Reflex save, which is highly unusual for save-or-suck effects.
Belker Claws: Respectable damage (up to 10d12!) over several rounds and it doesn't permit a save.
Bestow Curse: -6 to any ability score, or -4 to all rolls, means this spell is never inappropriate for the situation. You know what you can do with this one.
Blindness: Solid debuff that even opens the door for sneak attacks - best against low-Fortitude caster- and rogue-types.
Combust: 1d8 damage/caster level, no save - unless you count the Reflex save to prevent catching on fire. Excellent bonus damage at mid-levels.
Dispel Magic: This is one of the big ones. It'll run out of steam by level 12-14 or so, but it can be just the thing against a heavily buffed foe... assuming you can hit him.
Distract Assailant: Allows for a Will save and SR, but making an enemy flat-footed can open a lot of opportunities. Hold person is usually better, but this can pair nicely with Iaijitsu Focus.
Entice Gift: Compels the subject to give you what they're holding. Taking an opponent's weapon or spell component pouch can make them a complete nonissue.
Hold Person: Solid against fighter-types with low Will saves. Allows for sneak attacks and even coup de gras - whether you use this or blindness will depend on which save you want to target.
Lastai's Caress: Takes an evil foe out of the fight 75% of the time, checked each round. Oh yeah, and no save. Bordering on overpowered.
Mage Hand, Greater: Okay, this one will probably only work on kobolds or halflings, but the imagery is too funny not to suggest.
Melf's Unicorn Arrow: Deals decent damage, and has a nice chance of rearranging the battlefield a little (or pushing folks off cliffs.) Since the foe's probably going to be directly in front of you when you bull rush them, you'll get an attack of opportunity for them leaving your threatened space. Plus, no save or SR!
Poison: Needs to be cast by a druid in order to be 3rd level, but 1d10 constitution damage can be pretty vicious. Note that the save DC is not dependent upon the level of the spell but rather upon caster level. A 20th level caster with 30 Wisdom would necessitate a DC 30 Fortitude save.
Shivering Touch: You know it, you love it, your DM hates it. 3d6 Dexterity damage with no save - want to take out a dragon in one hit?
Fell Draining Sonic Snap: Because enervation isn't allowed but metamagic is.
Sound Lance: 1d8 sonic damage per level, with a Fortitude save for half. Hits casters hard.
Spell Vulnerability: Reduces SR by 1 per caster level, to a maximum of -15. Chances are, if you're using a spell storing weapon, you have a spellcaster around. Might as well make his job a little easier.
Stolen Breath: The subject is sickened for a full minute, with no save. It can take a full-round action to end the effect, but that just means free action economy for you.
Touch of Idiocy: Gives a penalty of 1d6 to all three of the target's mental stats. Good for lowering the DCs of casters and hopefully cutting out one or two of their spell slots. No save, too!
Vampiric Touch: A little extra damage, and even some pseudo-healing in the form of temporary hit points.
Vertigo: The subject takes a -2 to saves/attacks, and it must make a balance check each turn to move. Not amazing (especially since the DC is 10) but don't forget the one stipulation that makes Grease powerful: creatures without 5 or more ranks in balance are flat-footed as long as they're balancing.
I'm A Person, Too
An interesting (and debatable) application of spell storing is that it can store any "targeted spell" so long as it meets the requirements for level and casting time. So this means that any spell with a "Target: __" line qualifies, to prevent jokers from trying to cast blockade on their foes, I guess. Anyways, something you may not have considered is that Personal-range spells (with Target: You) therefore qualify. Now, most personal spells have beneficial effects, otherwise why are you casting them, so it's not like this will give you much more in the ways of hurting foes. Rather, this lets you share beneficial effects that party members would otherwise not have access to. Haste and bull's strength can be shared easily enough, but you can't cast a personal-range spell like bite of the wererat on the fighter - however, you can STAB it into him.
Chimping out on your allies just to give them buffs is admittedly cause for hesitation, but weapons that only deal nonlethal damage - such as the sap, whip or goad - are all still candidates for spell storing. Hitting your fighter for 1d4-1 nonlethal damage is probably worth it if you're granting him one of those personal combat buffs that are powerful but no wizard can make good use of, such as balor nimbus or fist of stone.
Even if your DM says no to the storing of "Target: You" spells, don't forget that there are still other targeted spells that are beneficial. A wizard might not have any healing spells, but he can still bonk a wounded ally with a +1 spell storing sap, delivering cure serious wounds. Since healing magic cures nonlethal damage as well as lethal at the same time, there's really no drawback so long as you can make the attack roll.
The latter-day darling of system abusers everywhere, Sanctum Spell is a feat with a reasonable premise but very dark implications. Basically, you designate a home base or "sanctum." A spell modified by Sanctum Spell that's cast in your sanctum is treated as if it were one spell level higher in all respects except for its slot. Not caster level, but effective spell level. However, such a spell cast outside of your sanctum is likewise treated as one spell level lower in all respects. The intent here is obvious: if you cast a spell in your Safe Space, it's more powerful, but once you're outside your comfort zone you fall apart.
Of course, we never much cared for the intent of game designers. The very relevant fact of sanctum spells is that being treated as one level lower makes caster level-dependent effects weaker, yes, but it also makes other effects treat them differently. A 4th level sanctum spell is treated as a 3rd level spell (that just happens to take up a 4th level slot,) meaning you could cast it into a spell storing weapon, so long as you aren't in your sanctum. Obviously there are much more degenerate ways of abusing Sanctum Spell (how does crafting a wand of Sanctum acid splash for a negative gold and XP cost sound?) but being able to store the likes of orb of Fire, lesser geas and even polymorph (!) will extend the usefulness of your weapon enhancement.
So you know you want to store spells, and maybe you've even picked a couple favourite spells for the task. There's one thing left to consider, though: which weapon should be your method of delivery? Lighting a guy on fire after you've already waled on him with a sword is a good way to ruin someone's day, but surely there are some weapons and techniques that can take particular advantage of casting a spell on hit.
The mancatcher, pincer staff, sharktooth staff, ritiik, scorpion claws, spiked chain, flindbar, harpoon. Each one calls for an opposed roll - either to grapple, trip or disarm - on a successful hit. However, if said successful hit also gives your opponent a penalty to their roll, you're making your job a whole lot easier. Storing something like heatstroke or bestow curse is an easy way to Make Your Feat Tax Work For You.
Several times I've seen online folks recommend the use of a spell storing net. Combining the dexterity penalty and touch attack of a net with a rider effect, perhaps shivering touch, certainly sounds attractive. Then you remember how to read and notice that spell storing weapons can only cast their spell upon dealing damage - something that nets simply don't do. Ditto for lassos. Even nonlethal damage qualifies, but no pain, no gain.
Oh, and before you start pondering the razor net, do remember that it destroys itself after one use. I mean, yes, it qualifies, but I'm not too keen on spending over 8000 gp on a glorified tanglefoot bag.
On the Flattening of Feet
Some weapons are best in specific circumstances - after those conditions pass, though, you're often left wishing you were holding something more utilitarian. For instance, the collapsing crescent fan and gnomish quickrazor both excel against a flat-footed foe, though generally that's only going to occur for the first round of combat. Storing distract assailant can help with iterative attacks. Or consider reach weapons, which are best when you have some breathing room. Putting melf's unicorn arrow into your spiked chain can keep your opponent from closing the gap with a 5-foot step.
Blade Magic Plus Magic Magic
I have a confession to make: I really like Tome of Battle. Giving options in combat to melee classes not only narrows the power gap between magic and mundane, it more importantly makes them so much more fun to play. You can make your sword-swinger act even more like a premade gish by combining maneuvers with stored spells, letting you stack conditions to devastating effect.
Performing the Mind Strike maneuver with a weapon storing touch of idiocy can hit a divine caster for a 1d6 penalty and 1d4 damage to their Wisdom score with one hit. Hamstring Attack plus shivering touch will immediately paralyze most foes. Emerald Razor lets you turn any stored spell into (for many of them, back into) a touch attack. The constitution damage from Bloodletting Strike or Mountain Tombstone Strike will lower your foe's Fortitude save for whatever spell immediately thereafter affects him. Bestow curse can't bring an ability score to 0, but it can when combined with any number of maneuvers that deal ability damage.
Adding poison to your weapon is another way to stack effects with a stored spell, but it's more expensive than refreshing a maneuver. Using all three together should debuff damn near anything that isn't homebrewed and looks remarkably like your DM's crush.
So as long as you have a steady stream of spell slots on hand, often in the form of a spellcasting party member with downtime, you can really step up your game when it comes to hitting stuff. All of these options certainly beat the hell out of +1d6 fire damage per hit.