My favourite, and definitely the fastest Peer To Peer software I've come across. I've added a link to the original programmer's webpage, but there's different versions of the client software out there.
It's different in a number of ways:
You open one copy of the program for every file you want to download
You can't download without uploading
Bittorrent hogs every spare bit of bandwidth - actually, once it gets up to full speed, you can forget about collecting email or surfing the net.
It's specifically geared to transferring large amounts of data in the least possible time. I regularly download 400-500Mb files in less than four hours.
Of course, it's started being used by software and video pirates, but as I posted here filesharing in general is a grey area.
Anyway, it's a great system. If you've got Broadband, it's well worth your time.
