Pandora, a tool created by the Music Genome project. Type in a song or artist you like, and more often than not, it'll bring up a list of music similar to the artist or song you searched for AND play it right there.
Another good source for online music is http://www.launch.com. It has lots of music videos, background info, and an online radio thingy.
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"Dear humanity, we regret being alien bastards. We regret coming to Earth. And we most definitely regret that the Corps just blew up our raggedy-ass fleet! (Hoo-Rah!)" -- Sergeant Johnson