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Pandora when I could use it at work. Streaming music all day and it makes suggestions based on my choices.
Firefox, as mentioned before.
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Similar to Pandora
Last.fm is good for discovering new music. Especially if you're an indie kid. It also has a larger music library than pandora featuring pretty much every piece of music on amazon, including classical, jazz, latin etc. Not as easy to use as pandora though, but not hard either.