Counterintuitively,
Avoid Weapon Special Abilities that Inflict Damage
If your PC has been taking feats like Favored in House (ECS 53), and Least Dragonmark (mark of finding (ECS 64)) (ECS 53), and Skill Focus (Bluff) (PH 100) so he can better make Seduce to Learn Secret checks while the other PCs have been specializing in monster murdering, and they realize you aren't contributing much in a fight, explain that you'll need their help. As a swashbuckler (who doesn't cast spells) you were going to need it anyway. That's not an insult but a fact of mid- and high-level play.
You'll need their help because the first weapon special ability you add to your rapier is spell storing (DMG 225) (+1 bonus). No other weapon special ability scales for damage as well as it does because it scales with your party, one of whom should be happy to refill your +1 spell storing rapier free of charge at the end of a slow adventuring day. If you must fill your own tank, take ranks in the Use Magic Device skill and load your +1 spell storing rapier using a wand of combust [evoc] (SpC 50) (2nd-level spell at caster level 10) (300 gp/charge) or a wand of vampiric touch [necro] (PH 298) (3rd-level spell at caster level 20) (900 gp/charge) or a wand of another spell that the Comments will inevitably suggest.
It sounds like you're in a game wherein sources of magic items are restricted. If you aren't, also consider the weapon special ability bloodstone (MIC 29) (+1 bonus), which is like the weapon special ability spell storing that only holds the spell vampiric touch but the touch is modified by the feat Empower Spell (PH 93) for free. Seriously. Then add the weapon special ability power storing (MIC 40) (+1 bonus) if there's a psion available; my unfamiliarity with psionics means I don't know what's best, but there must be something awesome and Eberron is psionics-friendly. Then virtually guarantee your attack hits by adding the weapon special ability impaling (MIC 37) (+1 bonus).
If you are confined to the Dungeon Master's Guide for your weapon, the only other serious choices for damage are the weapon special abilities bane (DMG 224) (+1 bonus), picking whatever the DM's throwing at you a lot, and holy (DMG 225) (+2 bonus) or, y'know, unholy (DMG 226) (+2 bonus) if you're an evil swashbuckler: a lot of your foes will be evil (or good) anyway; punish them for it. If chasing just damage buy the weapon special abilities magebane (CAr 143-4) (+1 bonus) and vampiric (MIC 45) (+2 bonus) if allowed.
The flat +5 damage from the weapon special ability collision (MIC 31) (+2 bonus) is the best you're going to get, but also consider, if it's permitted, the weapon special ability fierce (AE 96) (+2 bonus) that lets the weapon's wielder to take a penalty to his Armor Class up to his Dexterity bonus to add the the penalty's absolute value as a bonus to the weapon's damage.
Although others may disagree, most of the other damaging effects from the DMG and elsewhere inflict energy damage that a high level creature's energy resistance will easily absorb, employ effects high level foes are often immune to (e.g. ability damage and ability drain, critical hits, negative energy effects), or have saving throw DCs that are too low to be viable.
Effects That Don't Inflict Damage, But Take a Look
The weapon special ability dispelling (MIC 33) (+1 bonus) et. al. will make you popular with your group even if you aren't inflicting a lot of damage. The same goes for the weapon special ability revealing (MIC 42) (+1 bonus), which eliminates miss chances for your party; in high-level play miss chances are commonplace, and everyone will thank you even if you hit only once.
The elemental power weapon special abilities from DMG2 254-5 et. al. range from a +1 bonus to a +4 bonus and permit summoning of an elemental of increasing size based on the bonus purchased. Being an elemental-summoning swashbuckler may sound strange, but the value of minions with cool powers can't be overstated--ask any necromancer.
I don't like abilities that allow saving throws nor do I like abilities relying on critical hits, but the weapon special ability prismatic burst (MIC 40) (30,000 gp) is an exception to both. The flat cost means it's possible to add it to a weapon that's already full of other bonuses. The weapon special ability sending (LE 152) (+4 bonus) is also valuable, despite only activating on a natural 20 and a successful critical roll; a no-saving-throw teleport effect versus a high-level foe is more likely to remove him from battle than almost any other effect, although by the time this is avaialable most foes should have a way of returning to the battlefield shortly.
I've not heard of a DM actually allowing the weapon special ability smoking (LD 180) (+1 bonus), but I suggest it because it exists. It's a defensive ability, but a spectactular one, and defensive abilities keep you alive long enough to use offensive abilities. The weapon special ability spellblade (PG 120) (6,000 gp) is just as cool; pick a targeted spell the DM loves, ignore it, and even huck it back at the caster.
The weapon special ability Keeper's fang (ECS 266) (+4 bonus) renders creatures you kill with the weapon unable to return from the dead via the spells "resurrection, raise dead, reincarnate, or similar effects"; the weapon becomes evil and makes you look like a jerk to some folks, but this is such a powerful ability in any typical Dungeons and Dragons campaign that its utility can't be ignored.
Don't Neglect Accessories!
A broken weapon made from aurorum (BE 38) (+4,000 gp) can be made whole as full-round action, but a weapon made from riverine (Sto 128) (+2,000 gp/lb.) is just immune to damage.
A weapon made from morghuth-iron (AE 14) (+4,000 gp) is constantly naturally poisonous, but the poison's saving throw DC is low, the weapon's wielder takes a -1 penalty to attack rolls and damage rolls with the weapon, and many high-level creatures are straight-up immune to poison. This is, however, far cheaper than many effects, and there's no use limit.
A weapon with the template pitspawned (DMG2 278) (+1,000 gp; 0 lbs.) grants the wielder a +2 bonus to critical rolls.
A wand chamber (Du 34) (100 gp; 0 lbs.) is a space in the weapon for a wand which can be used without drawing the wand; perfect if you've ranks in the Use Magic Device skill. A hilt hollow (Du 33) (300 gp; 0 lbs.) is a space in the weapon for a potion which must be extracted but it's easier than normal. Get both. An oil chamber (Du 33-4) (1,000 gp; 0 lbs.) is expensive, but can hold an oil of greater magic weapon [trans] (PH 251-2) (3rd-level spell at caster level 20) (DMG 230) (3,000 gp; 0.1 lbs.) or ghostoil (AE 34) (50 gp; 1 lb.). A triple weapon capsule retainer (CAd 121-2) (450 gp; 0 lbs.) holds 3 alchemical weapon capsules (CAd 122), which do everything from more damage to silvering your weapon on the fly.
You'll want a weapon with at least a +3 enhancement bonus (not just a +1 weapon with at least 1 additional +2 bonus or 2 additional +1 bonus weapon special abilities!) if planning to attach the most powerful augment crystals (MIC 221). Start with the crystal of return (least) (MIC 65) (300 gp; 0 lbs.) now if you've not the feat Quick Draw (PH 98) or another way to draw a weapon as a free action. Purchase more augment crystals as needed for the campaign.
Suggestions
All bets are off if the campaign allows more sources than these.
- A +1 bane (humans) holy spell-storing adamantine rapier (53,320 gp; 3 lbs.) if you can only use the Dungeon Master's Guide.
- A +1 bloodstone collision dispelling impaling prismatic burst spell-storing vampiric adamantine rapier (195,320 gp; 3 lbs.) if you can use the Dungeon Master's Guide and the Magic Items Compendium. Perspective: This weapon costs nearly two tons of gold.
Best Answer
So, You Want to Keep One Hand Empty
Spellcasting is very close to the only good reason a creature keeps a hand empty, and even then taking a free action to keep one hand on a weapon the creature usually uses two-handed still permits the creature to cast spells, as does toting (but not employing) a buckler. Besides spellcasting, though, only a handful of reasons exist ever to have a hand empty of a weapon or shield.
You want to steal stuff. "[T]o snatch an item worn by the target" can be a disarm, but, instead, "[i]f you try to take something from another creature, you must make a DC 20 Sleight of Hand check to obtain it" (PH 82) as a standard action. Some feats improve the Sleight of Hand skill option--the best are City of Stormreach's Master Pickpocket (95) and Dragon Compendium Volume 1's Cutpurse (95). The interesting thing about the Sleight of Hand skill check is that it can be performed as a free action by taking a -20 penalty, which is a lot, but until the DM limits your free actions, once you've a Sleight of Hand skill bonus of +20 or higher (and can avoid the attacks of opportunity that both feats eliminate) you can just steal all of every adjacent opponent's stuff... by taking, potentially, hundreds of free actions and dropping things after swiping them. This becomes even funnier if you've reach. This tactic is even tacitly encouraged by the feat Master Pickpocket, which reduces the free action Sleight of Hand check penalty from -20 to -10.
This is actually a good reason to have an empty hand as a non-spellcaster. It is interesting, works against any gear-laden foes, stays relevant largely until the game ends, and is an appreciable combat debuff. Your DM may start limiting you, though, especially if you're doing this at low levels when the spellcasters aren't yet controlling reality, so you might want to sandbag if you can do this at, like, level 5, and--very seriously--folks will trap their gear after rumors circulate that you make enemies naked then dead instead of the reverse.
You've something important in that hand that's not a weapon. Wands are a good choice, but your main weapon should already have a wand chamber (Du 30, 34) (100 gp; 0 lbs.). And so should your gauntlets (PH 117-8) and your spiked armor/armor spikes (PH 116, 123-4) and, if using Complete Scoundrel, your boot blades, elbow blades, knee blades, and sleeve blades (109-10). In other words, you've enough wands all the time. Here are some other choices.
Note that even a light shield "lets you carry other items in that hand" (PH 125), therefore many options below are also permitted while employing a light shield or a buckler (PH 124).
You want to disarm a foe and keep his weapon. The disarm maneuver is difficult, but "[i]f you attempt the disarm while unarmed, you end up with the weapon in your hand" (PH 155), which, in addition to looking cool, in my experience also renders many warrior NPCs useless. I have never seen the disarm special attack used in a campaign to such a degree that any creature actually uses a locked gauntlet (PH 124-5), but a DM confronted with a master disarmsman will probably have the NPCs start using it. The feat Snatch Weapon (Song and Silence 40--I know!) lets you end up with the weapon in hand after a successful disarm attempt with a weapon and stab the dude from whom you took it.
But, yeah, you really should weaponize that empty hand. If your feats are spoken for, gloves of the balanced hand (MIC 105) (8,000 gp; 0 lbs.) allow the wearer to fight as if he had the feat Two-weapon Fighting (PH 102). If that hand still must be empty, get a fanged ring (DM 101) (10,000 gp; 0 lbs.) or bracers of striking (Mag 110-1) (1,310 gp; 1 lb.); the bracers can explicitly give your unarmed strike weapon special abilities, so go ahead and add to your hands the weapon special ability defending (DMG 224) (+1 bonus) or whatever.