SomaFM
SomaFM focuses on genre-based internet radio channels.
What it does
SomaFM streams internet radio channels organized by style.
When to use it
For a long work session, try one channel and judge whether its pace fits.
Visit SomaFM official site
A Soft Murmur
A Soft Murmur focuses on mixing rain and other ambient sounds.
Audius
Audius focuses on artist-uploaded and shared original music.
Bandcamp
Bandcamp focuses on discovering and buying independent music.