Webisodes

web-i-sode - n. - It's not in Webster's yet, but it is on Wikipedia. Basically, it's an episode of a television show made specifically for distribution on the web (www or World Wide Web).

Heroes

A very good example (and one of my favorites) is the Heroes graphic novel. It fills in the gaps, gives background, and extends the story of the broadcast episodes. You can view them in static or animated form, or download them in printable pdf format. There are 72 chapters now. Check them out!

Monk and Psych

On USA. Also see full episodes of each. And Burn Notice, too.

Dexter

Three webisodes of The Dark Defender, Dexter's "superhero" (?) alter ego. You can also see a toned-down version of Dexter on CBS starting Feb. 17.

Battlestar Galactica

(Surprisingly, they are still available. Other, more recent webisodes on Sci Fi Pulse have disappeared.)

Chuck

The nerd super-spy on NBC.

Mythbusters Raw

Not SF ( it's classified as "documentary," not reality), see outtakes, extra footage, and original stories. 80 clips available. (Mythbusters Uncut, Experiment Extras, Busting Buster, much more.)

....and many more. They come and go (can't find the Eureka or Razor webisodes anymore - maybe on You Tube) so keep checking your favorite shows' network website for new content. They also usually talk about them during commercial breaks (if you don't DVR over them).