Mountain Hammer is a nifty little maneuver out of Tome of Battle that adds 2d6 damage to a melee attack and ignores both hardness and damage reduction for the attack. It's part of the Stone Dragon discipline (note: it's got a less-powerful cousin in the Devoted Spirit discipline). You can pick it up with three levels in any of the initiating classes (Warblade, Swordsage, or Crusader) or via the Martial Study feat as a sixth level anything else (note: to meet prerequisites you may have to take Martial Study more than once).
Shards of Granite
Also from Tome of Battle, Shards of Granite is a tactical feat that lets you ignore damage reduction and hardness while fighting if you meet certain conditions. You'll need the Stone Power feat to qualify for it.
Energy damage (fire, cold, electricity, acid, sonic, negative energy, etc) is not subject to damage reduction, and is instead subject to energy resistance.