If you've got a spare box on the net, you can use the Shoutcast stuff. You can DJ from another computer (or the same one) using just standard WinAmp with an extra plugin installed...
The tools are übar-decent and yet really really cheap, where 'really really cheep' means 100% free.
You can run a server on linux-capable stuff, but you'll have to DJ from a windows machine.
Go to
http://www.shoutcast.com/download/ and check out both the 'Be a DJ' and the 'Be a server' links.
(If this is the right sort of thing that you're after, but you're not using Windows (yeah, I've seen your mac)/you don't like winamp for some strange reason, google for IceCast. That's more of the same only more open-source.)
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