http://tvtorrents.com - Organized by series, season, episode, and quality - SD, HD, DVDRip, BDRip, .mp4, .avi, etc. They have entire-season bundles of just about every show you can imagine (Bosom Buddies? Yes. Dexter's Laboratory? Good Eats? Yes.), and new episodes usually appear right after they air.

You can also set up a personalized RSS feed of your favorite shows and connect that to some clients that will automatically download them as soon as they air.

Normally you need an invitation to join but occasionally they have open registration (like right now) where they let anyone sign up. They are very strict about ratios; if you drop below 1:1 you'll be shut off. If you're only downloading the newest stuff it can be tough to keep your ratio up (because so many are seeding). The best way around that is to either donate or buy a t-shirt, either of which gives you 3X upload credits for a year.