CBS has just launched webisodes of Star Trek: The Original Series seasons 1, 2, and 3. They're all full-length features, all you have to do is sit through an occasional 30 second ad before the launch of the clip.
Also available at the time of writing: MacGyver (season 1), The Twilight Zone (seasons 1 and 2), Hawaii Five-0 (season 1) and Melrose Place (season 1).
If you missed these shows on TV, here's a chance for you to catch up for free!
(CBS also has select full episodes of more recent TV shows, just click on the "Shows" link on top of the page and select the TV show you'd like to see - some have full webisodes, some don't.)