Public Sites - easy to register or just plain open
Demonoid - Very good varied content site with open registration: Software, music by the album, films in various languages, some TV
Mininova - a great open site - No registration. Lots of popular content. No
MVGroup - Open Registration - An amazing site run out of the UK by people who are very serious about documentaries especially BBC and PBS. This is my favourite site on the internet. Great user posted section. Official MVGroup releases are always superb quality rips and interesting and informative content.
ConspiracyCentral - Open registration site. Some niche content Metaphysical, Religious, Paranormal
DigitalDistractions - No Reg. required. Good For Some Specific TV Shows: Nova, Iron Chef, etc.
The Three Sisters - You must be invited to these sites and you must maintain a reasonable upload ratio or you get banned. Invites are hard to come by. Because so many people are desperate to upload, download speeds are amazing and the content is very complete.
BitMeTV - Pretty much every mainstream US television show - downloadable by season.
BitMe - Educational Content - some art, books and audiobooks, documentaries
UKNova - Has all the Current UK television shows.
Torrent Searches - if you know what you want and are willing to download from random people in random places and are not afraid of diseases, give this a try after you have searched your trusted sites.
TorrentSpy - A Torrent Search Engine
isoHunt - Another Torrent Search Engine with an interesting interface
Torrents are not a secure a P2P filesharing method unless you have trust in every peer on the tracker. Ultimately, users who really want a secure but content rich filesharing experience will have to evolve into some sort of a Social Networking automatic subscription service. This has some immediate practical elements, but cannot be a dominant strategy until hard disk space and bandwith become significantly cheaper. More on that later...
Feel free to comment with other sites I should have mentioned
Disclaimer: I do not endorse violating copyrights. Use these sites at your peril. Copyright holders unions are actively suing people, so be careful out there. Share and download only with permission of the copyright holder and after having consulted with a lawyer about any such activities.