Are you looking for specifically classical music, or just instrumentals?
I personally concentrate better when the room is silent, but when I feel the need for some ambient music for times of extra stress, I turn to "The Terrys: Through It All." Primarily guitar and strings music, it combines all sorts of genres to make some amazing comfort tones.
Quote:
An instrumental album of original music creating a soulful blend of fingerstyle guitar and violin with folk, Celtic, bluegrass, country and classical texturing.
link to listen to clips from a few tracks 
