The blog Pecuniarities has a great post with all the Web sites where you can watch TV for free. For free!
They include Hulu.com (a great all-around site), TV.com (many shows are in HD), Fancast.com (has videos not on Hulu, like old episodes of Friends), Joost.com (has content from Showtime and HBO) and TidalTV.com (good selection but quality is not as high).
Makes you want to kill your cable service, right? Read the original post for reviews of each site.