Azureus: 3d View
Except for its nasty tendency to crash unexpectedly (great strides have been made, but it still does it once in a while), Azureus is pretty much the BitTorrent Client. My favorite thing about it (and this seems to be a pattern of open source projects) is its extensibility. There’s everything from a Flag plugin (to get a kick out of how international piracy is!) to a Web (HTML+JS+CSS) UI to the program.
But my favorite plugin is far and away 3D View — a dense, beautiful 3d representation of the torrent process (really, just the standard swarm graph writ 3d). Like the 12/60 clock, it comes with no instructions but it doesn’t need them. If you’ve read anything about how torrents work (and you should!), everything will fall into place after some staring. Its pure infosight — real-time infoporn of the best kind1.
fn1. Which reminds me: I heard somewhere today that “there is no such thing as bad porn, there’s only better porn.”