Here are a number of free online sources of music and background noise that are particularly good for high-concentration tasks.

• SimplyNoise – Pure white, pink and brown noise (noise)
• Rainymood – Stormscape  (noise)
• musicForProgramming(); – Ambient mixtapes (music)
• SomaFM’s Groove Salad playlist – Ambient radio (music; playable via browser popup)

And here are a number of (not free) albums I find particularly good for this purpose.

• The Album Leaf’s In a Safe Place (Amazon link)
• Brian Eno’s Music for Airports (Amazon link) and Apollo (Amazon Link)
• Jon Hopkins’ Contact Note (Amazon link) and Opalescent (Amazon link)

This isn’t a comprehensive list by any means (that sort of thing can be found on Lifehacker, as you might expect), but over the past few years, I have tweeted and blogged about this a bit, so I’m simply tying up some of the odds and ends in one place.