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Shadow Blade, from Tome of Battle, requires a level in Swordsage or 2 other feats and gives dex to damage in addition to str, but only with certain weapons, all of them save for the spiked chain light weapons (dagger, kama, unarmed strike, some others I forget...)
In Races of the Wild, there's an Champion of Corellon prestige class for Elves that lets you add dex to melee damage, too.
EDIt: Ninja'd.
Also, this thread USED to be awesome, but wotc's cluster of a site reorganization seems to have left it very hard to read.
There is a book called Feats, put out by Alderac Entertainment Group, it is D20, but I think it is 3.0 edition. In it is a feat called Superior Finesse, which does exactly what you are looking for. I do not know if the feat can be found in any WotC books, but if your DM allows other books like it then you should be good
Hmm, Drow of the Underdark has a fighter variant that gives you dex on damage if the target is flanked/flatfooted (in addition to +2 init. It does cost you your heavy armour proficiency though..)