Things with interesting music, but without lyrics. Nine Inch Nails' "Ghosts I-IV" is a good start, as is the NIN ambient 'music' of the sound-track to the original Quake.

The Canadian indie label Constellation has a number of artists that make good coding music, including Godspeed You! Black Emperor, Thee Silver Mt. Zion Memorial Orchestra, and Do Make Say Think.

Other relevant artists include: Mogwai, Boards of Canada, Dirty Three, Explosions In The Sky, LCD Soundsystem, Monolake/Robert Henke, The Necks, Ratatat, The Flashbulb, The Samuel Jackson Five.

There's a few particularly good movie soundtracks I like, as well: Koyaanisqatsi, Le Fabuleux Destin d'Amélie Poulain, and Blade Runner.

Another good source of quiet, interesting, music is videogame soundtracks. I'm particularly fond of the soundtrack to Ōkami, and the OCRemix tributes to Donkey Kong Country, Super Metroid, and Doom: http://ocremix.org/albums/?&offset=0&sort=nameasc