http://www.spotify.com/us/hello-america/comb/I'm actually a little surprised nobody has posted this yet.
Spotify is the best new thing for your musical playlist. It's essentially taking itunes format, and enabling you to use the internet to play songs. With the ability to find related artists, share playlists, and find more music by most bands, it seems somewhat similar to pandora. However, you get to select your own songs, and in the weeks I've been using it, I've so far received only two audio ads, which destroys pandora.
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Relatively ancient and inactive
When I tried it, I got an audio ad after four songs. Since I pay for Zune Pass, which is pretty much Spotify Premium for Windows/Zune/WP7, I just uninstalled it right there.
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I like www.thesixtyone.com for finding new music much better. Sort by genre, a lot of unknowns, and usually just damn good music.
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Similar to itunes? One point against it already. Use the internet to play songs? ... I have my computer for that. Find related artists - well, does it work better than last.fm for that? Last.fm has been pretty useful in finding extremely rare stuff for me in the past, so...
I dunno, man, seems like I can do all of the above without ever worrying about ads, bandwidth, or anything fishier/fisherpricey.
Show them your butt, and when you do, slap it so it creates a sound akin to a chorus of screaming spider monkeys flogging a chime with cacti. Only then can you find your destiny at the tip of the shaft.