Hulu is a video-on-demand website, owned by NBC and FOX, that allows you to see entire episodes of your favorite shows like Heroes and Nip/Tuck, as well as entire movies.
Hulu was designed to compete with YouTube by offering free online video, and along with the iTunes Store, by offering premium video content. The only drawback is you have to watch a few short 15-second commercials that are shown in place of those longer commercials as shown on regularly broadcast TV.
Hulu seems to work best with short clips (like Saturday Night Live) rather than with longer videos. It faces other competing video-on-demand websites like CBS-owned TV.com and Joost (which recently eliminated its software-based viewing model).