Shoutcast.com is a great source for stations, and mostly in the category of Alternative Rock, cause there are tons of good stations. Internet radio is pretty sweet because most stations now adays are coming in 128k quality (although not all that great, its MP3-equavilant). I just hope it doesn't get shut down anytime soon.
I'm a huge SHOUTcast listener, I listen to it about 90% of the time. I listen to a variety of stations, from WOLF FM to the RadioStorm Top Pop 40, Classic Rock (favorite), Hip Hop, Groove Salad, JJ McKay's Hot Hit Radio, and Digitally Imported. Good stuff.