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

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

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.