[enigmango]

A few folks have mentioned white noise already. I used to work in an open-office layout with about 50 people, and I found it was much easier to mentally cancel out the chatter with consistent, light, inoffensive background noise than to try to create silence.

http://mynoise.net was great, and I only had a $10 pair of in-ear earbuds. I haven't used the site since changing jobs, but I remember liking Rain On A Tent, Wooden Chimes, and mumbly-voice environments like Laundromat and Airport Terminal.