Hope it's OK for me to take the chance to plug my own ambient noise mixing app:

http://asoftmurmur.com

I find ambient noise (at a low, safe volume) to be helpful for concentration.

Oh awesome, I sleep with this on in the background most nights.

Music for Programming has some interesting stuff, but personally, I need to systematically avoid anything with lyrics to have a hope of being productive while listening.

...?

I could be wrong, but the vast majority of MfP doesn't have lyrics. It's all atmospheric, vague, kinda droning music.

http://musicforprogramming.net/?c=aesthetic

Groove Salad / SomaFM or Digitally Imported / Chillout Channel.

I use http://focusatwill.com/ and it helps increase my concentration.

http://noisli.com is really great. Mix your own ambient sounds.