While browsing ISTE's website I came across a link to PBS' American Field Guide. There are over 1400 streaming video clips in either Real Player or Windows Media Player format.
You may browse the categories-- animals, ecosystems, human history, livelihoods, earth & space, plants, public policy and recreation-- or conduct a keyword search and even limit your search by state.
In the Teacher Resources section you will find several "Units of Inquiry" intended for middle school or high school students. Topics include: fires, floods, flowers and plants, insects, landfills, mammals and others. Each unit includes lesson plans, content standards, downloadable documents, and links to the featured video clips for each activity in the unit.
The streaming videos pop up in a small window, but right-clicking on a Windows Media Player video will allow you to enlarge it to full screen. (I don't have Real Player, but I assume it will also enlarge the video.)
If your school is fortunate to have a unitedstreaming subscription you may find similar content in their collection, but this free service from PBS is definitely worth a look.
P.S.- I just discovered you can email a link to a specific clip! Teachers with a classroom website would be able to copy the URL and link directly to a video clip for students to access from home.