Comrade F
A talented artist with quite a lot of enchanting music. Go over there and show him some love.

[New!] Grooveshark
My favorite music-streaming service. Find almost any song you can think of, listen to it whenever you want, and feel content in the knowledge that the artists are compensated every time. You can do it all for free, but consider showing your love for the site (and getting some snazzy VIP benefits) by giving 'em a few dollars now and then.

Kirill Pokrovsky
Divine Divinity and Beyond Divinity have some of the most beautiful computer game music I've heard. Apparently the composer is as generous as he is brilliant, and the full albums are available for free on his site. (You can listen to some sample tracks here and here to see what they're like.)

Live365
A vast index of online radio stations. You do have to sign up, but they never send you spam, and even a free account gives you unlimited listening time.

OverClocked ReMix
This site has more video game music remixes than you can shake a stick at. The songs come in just about every genre, and it's a great way to relive some of your favorite gaming memories.

Positively Dark
Positively Dark is positively divine. The tracks "Howl" and "Mermaid" are especially recommended.

The Sixty One
It's a music site which is also a game. Discover new artists, enjoy new songs, and... level up!