I use:
www.uknova.com - Excellent source for UK programs, nothing retail is allowed on the site. You have to maintain a ratio to remain a member in good standing. Nice community, my favorite.
www.demonoid.com - Currently open for membership. Has a little bit of everything, and I've found some great Doctor Who stuff there.
www.piratebay.org - Again, a little bit of everything. Sometimes they have stuff before anyone else does. A more international user base, that's where I got my ISO files for the Swedish coverage of Live 8 last year. Cool stuff.
www.tvtorrents.com - Strictly television shows.
www.mininova.org - Also has stuff before anyone else does. A lot of private trackers post through them, too.
www.myspleen.org - Just recently discovered this, and I've tagged a lot of stuff I'd like to download. Can't speak to the quality yet, but some stuff I'd like to have just to watch, and some other traders have recommended this site.
As for clients, I played around with them, and found for my setup, uTorrent worked best for me. If the standard BitTorrent engine doesn't give you good speeds, try other things, like Azureus, uTorrent, etc. If there's a site you want to use regularly, check the FAQ to see what clients they don't recommend - you'll get lousy ratios with clients that don't communicate properly with the site.
