BitTorrent

BitTorrent Guide & FAQ
BitTorrent Client Download [basic]
Azureus [advanced users]
Links to BT Tracking/Search Sites
Better List of Tracker Sites
SuprNova [now Exeem but has Cydoor spyware; use Exeem Lite instead]
BT BOT
TowerSeek [prototype]
Loki
Throughput
Pirate Bay
Movie Definitions
Tweak XP Ports for BT
Blog Torrent

For further insight read:
Wikipedia
PowerPoint Overview Berkeley SIMS
Why BitTorrent is Wave that can't be easily stopped!
The Next Generation P2P - BitTorrent
The Pirate Internet by Ed Felton and The Shadow Internet by Jeff Howe at Wired January 2005
P2P Traffic Analysis