Mythic Examplar (Complete Champion p.88)
A character who takes the 10th level of the Mythic Examplar prestige class who follow the Sunyartra paragon can ignore 15 points of any damage reduction, except epic damage reduction. This ability is permanent and constant.
You can additionally overcome epic damage reduction simply by wielding a +6 or better weapon.
Alternatively, the spell vulnerability (Draconomicon p.115) reduces a creature's damage reduction by 5 points, and another 5 points for every four caster levels beyond 11th. It can only reduce to a minimum of 5.
After consulting the feat index for references to damage reduction, I'm reasonably certain there's no feat which reduces damage reduction.
There are feats which will let your attacks ignore specific types of damage reduction: Aligned Attack (Expanded Psionics Handbook, p.41) and Improved Smiting (Complete Divine, p.82) can bypass alignment DR; Penetrating Damage Reduction (Epic Level Handbook, p.63), following the 3.5 update, bypasses one material type of DR and can be taken multiple times, and Improved Ki Strike (Epic Level Handbook, p.58) bypasses on unarmed attacks only, although perhaps there is a prestige class somewhere that allows Ki Strike with weapons.
Other partial solutions
A sword called the Nightblade of Arvandor (Book of Exalted Deeds p.115) lets you make ranged attacks which ignore all damage reduction as they're technically magical force effects.
The Infused (Lantern Archon) (Dragon #321), at 7th level of this prestige class, can shoot light rays which ignore damage reduction of any type.
The Forsaker (Masters of the Wild) can ignore damage reduction of any form he himself has. However, that was a 3.0 book, and the Forsaker was not officially updated to 3.5, where the damage reduction rules are very different.
The Ring of Elemental Command (Dungeon Master's Guide) allows a weapon to bypass the damage reduction of any creature from the elemental plane to which it is attuned.
The Golembane Scarab (Dungeon Master's Guide) allows a weapon to ignore the damage reduction of golems.