LaunchCast for me.
They used to be ad-free, but now Yahoo has bought them. Still only about 1 ad per hour. Servers are always up as well.
Whats nice is the ability to X out a lame song/band/album and never hear it again. You rate songs and that determines how often you hear them.
Also you can pick from many genres of music.
If you have a Yahoo account, you are all set. Try them out.
Cheers..........
Hanover