There are lots of really good educational videos on the internet. Most of the ones I've found relate to science, but there are videos available for other subjects as well. Here are the best sites I've found:
The Annenberg/CPB site has a lot of great streaming videos that could be used across the curriculum in just about every subject. The site requires you to register, but registration is free. The list of hosted videos includes some relatively famous educational series (including Mechanical Universe and The World of Chemistry)
The American Field Guide has many great environmental videos that can be used in any life science curriculum. There are also some geology, earth science, and archeology videos available.
The Vega Science Trust hosts videos of lectures, roundtable discussions, and workshops that present the cutting edge of science. Some would be appropriate for students while others just provide great education for teachers. The site also has some educational resources to go along with some of the videos.
101Science.com has lots of great links for science videos. Some of the above links came from there. This site hosts a lot of other really good stuff as well, but I haven't had a chance to surf through it all yet.
At some point in the future I'll post some instructions on how to capture streaming videos so you can view them at your leisure or be sure to have them if the internet goes down the day/hour you plan to show them to your class.