Usually chilled electronic stuff that's pleasant to listen to, and not so backgroundy I completely tune it out, but not too busy either. Recently I've been loving a 3CD set I got with:
Concise - Revive
Flaque - Mindscapes
X_Dynamics - Regular Loader Routine
..ambient pop that functions well as a study aid. Stars Of The Lid, Synaesthesia and Biosphere work well for this.
Otherwise, Detektivbyran and Samuel Jackson Five are excellent music for doing anything to - you can either concentrate on how excellent they are or sideline them and focus on something else.