Deadly Defense( Complete Scoundrel, p. 76)
You are at your most dangerous when forced to protect yourself.
When fighting defensively, you deal an extra 1d6 points of damage with any light weapon or with any weapon to which the Weapon Finesse feat applies (such as a rapier, spiked chain, or whip). This feat's benefit applies only when you are unarmored or wearing light armor and not using a shield.
If you have the Combat Expertise feat, you also gain the benefit of Deadly Defense when taking a penalty of at least -2 on your attack roll from that feat.
A fighter can select Deadly Defense as one of his fighter bonus feats (PH 38).