If you're just starting to watch TV online, Hulu is a great starting point. Hulu has a diverse collection of advertising-supported TV shows and movies. From the latest episodes of the Simpsons to classics such as Alfred Hitchcock Presents, there's something for everyone. If you'll be watching with kids, you may want to read A Parent's Guide to Hulu. Hulu requires Flash 9.0, but no special software. Note: Some of the entries are only clips, so make sure to look for full episodes/movies.
YouTube has gone beyond user-submitted content and is now another option for you to watch TV online. Most of the full episodes are more classic programming, so visit YouTube to enjoy favorites such as The Addams Family, Fantasy Island and Star Trek. No special player required. You may also want to read Using YouTube with Your Kids and Great YouTube Videos for Kids.
ABC has full episodes of TV favorites such as Grey's Anatomy and Desperate Housewives, as well as special features like interviews. Some of the episodes are in HD. The ABC shows require the download of a special media player.
The NBC website has full episodes of The Tonight Show, Community and more than 20 others (click on "Watch Video"). There are also short summaries and special features. You can watch online without any extra plugins and some episodes can be downloaded as well.
Visit CBS to find full episodes of shows such as Numb3ers, CSI and Two and a Half Men. There are also some great what-happens-after-they're-voted-off scenes from Survivor. You can watch the videos in your browser without a special download.