Documentary Dish is a great site with videos covering space, nature, technology, history, the environment and health. Videos range in length from 2 minutes to a full 50 minutes and longer.
The videos would work great as an introduction to a particular unit or lesson.
Check out the Galapagos Volcanoes video in the nature section. It could be used to discuss volcanos, the Galapagos Archipelago or as an introduction to species diversity. It's a quick (5 minute, 30 second) but beautiful and engaging video.
The 55 minute video about the Wright Brothers in the history section is a great way to illustrate how experts and innovators work through problems and teaches about the Wright Brothers.
Lastly, check out the shows section. A dozen shows with multiple episodes covering topics about space.
I found a couple broken links, mostly videos that have been removed from youtube but overall, this is a great repository of videos that can be used with a number of different age groups in a number of different content areas.