Pretty huge. As far as hacking goes, you can't really beat a 1-2 hour long seamless mix.
Anything by DJ Decryption, he does intellectual rap that's interesting to listen to.
Pink Floyd, specifically the Gilmour stuff.
Minimal electronic album that places you into a rhythmic trance, yet doesn't distract your attention away from the task at hand.
Skulltaste - Mux Mool.
World Class Listening Problem - Don Caballero.
Hitman soundtracks by Jasper Kyd.
I came across The Sight Below and Mux Mool from the excellent ISO50 blog. Browsing for music through the site can be a real time-sink but you'll definitely find something new. http://blog.iso50.com/category/music/
and for some other instrumentals: Andrew Bird's Useless Creatures, any non-vocal track by Broken Social Scene, Nosaj Thing (anything he makes, really), and James Blake.