I'm a fan of Last.fm
You install their little app (its not actually a plugin for iTunes, but it works with it) and it keeps track of what you listen to. You can add friends etc and see what they've been listening to...
But the really cool part is that you build "neighbors", people who listen to similar music. They have some kind of liscensing worked out, so you have what's called "Neighbor Radio" where you can listen to music that your neighbors are listening to, provided they have it liscensed and online to stream. You can also enter the name or an artist and tell it to play you similar artists. For the radio features, you have to download their stand-alone player, but I don't think its that big of a deal.