Where can I listen to free music online?

I want to listen to music online while I do some school work. I don't want to download the songs, only listen to them in my computer.
I know about some websites but they're not free.
Any good websites for free online music?

I recommend immem.com, is a very good free service with millions of songs and a very good quality.

I use last.fm personally. It stores the artists that you like to listen to and plays stuff similar to your tastes.

And yes, totaly free! All you need to do is sign up.