I go for stuff that doesn't have words in English (or if it does, they're few and far between) -- these days it's mostly electronica. I like The Orb and Massive Attack (although Massive Attack fails the not-English), but my absolute favorite "work to do" music is two albums by Mixmaster Morris and Pete Namlook, called "Dreamfish" and "Dreamfish 2". (It's also my on-airplanes music.)

Amon Tobin is another good one: instrumental quasi-electronica with a lot of motion. I'm also fond of Christopher O'Riley's piano reinterpretations of Radiohead and the London Philharmonic's version of Led Zeppelin. Other groups on my "background" playlist: Deepspace, Solar Fields, Byron Metcalf, Stars of the Lid, Al Fabris, Hammock, Aes Dana, Asura, Baird Hersey & Prana, Brian Eno, Biosphere, Steve Roach, and Thom Brennan.

(All of 'em are on the iTunes music store; I found most of them through Genius results.)