A friend of mine got me hooked on Yahoo's LAUNCHcast. LAUNCHcast is a customizable online radio station that attempts to play YOUR style of music all the time.
Here's how it works - when you sign up (it's free), you go through a music selection process and indicate your favorite music genres and artists. LAUNCHcast then takes all of your information and builds a radio station with your favorite artists or other artists similiar in style. As each song plays, you can rate the song/artist and whether or not you like it. The more data it collects the more personlized your station becomes.
On my playlist: Coldplay, REM, Norah Jones, BNL, John Mayer, Moby, Switchfoot...