(Updated. Originally posted 11/2009)
WatchKnow is a site set up by the creator of Wikipedia. The site collects all the best free educational videos for students and indexes them for easy searching and you can watch them on the site.
The videos are links and are actually hosted on their home sites, so some of the videos will be blocked by internet blocking software.
The categories include: math, science, social studies, history, language arts, and much, much more. There are even some of the School House Rock videos on there.
The best part about WatchKnow is that it has done the work of sorting through the millions of videos on the web to find the ones that are good for education. Do a search and see how easy it is to use and find great videos for your classroom.