Demonoid is a pretty well known tracker. But for all intents and purposes, it is pretty good and the files on there are pretty usable.
Isohunt is a pretty good torrent search engine. It tells you where the torrent is from, and tracks some Demonoid torrents as well. I use this particularly because it has very fast load times.
Pizza Torrent is a new torrent search engine. The category selection features are better than Isohunt. It is pretty fast too, and seems to pick up more torrent sites than Isohunt.
There are legal issues with using Torrent sites that I will not discuss. Use your digression accordingly.
Updated April 13 2008