Game sound tracks may be a good option, because to they are designed to be in the back ground, and not to be grating on the ears even after much repetition.
The sci-fi racing series WipeOut has some great tracks for fast-paced instrumental background music. Picks me up every time.
The Portal 2 soundtrack has a good mix of fast and slow/eerie, which I find great for working through things... though I may just associate it with problem-solving because of the nature of the game.
For calmer, more meandering listening, I've got the Morrowind soundtrack. Though really any recent Elder Scrolls game has some great wandering music.
