If you don't know what the BitTorrent protocol is, Read This.
If you'd like help finding the best BitTorrent Client, Read This.
Massive Torrent Search of ALL major networks, along with a vast array of rather obscure trackers.
List of sites searched, in case you (for some reason) only want to use one.
Feel free to suggest any other trackers you feel I may have missed in the comments section.
Find Your Torrent, Or Else.
Here is a list of invite-only torrent trackers: http://hubpages.com/hub/invitetorrents
These are open for registration: http://hubpages.com/hub/openregtrackers