@sean Doesn't archive.org have a wealth of public-domain videos? Perhaps you could add them.
You might want to double check with some of the content, I remember seeing the old Charlton Heston version of Ben Hur on there, apparently added by Torrent. I'm not sure if it's in the public domain, but I don't think it is.
@sean Vimeo, perhaps?
The trickier bit is finding content made by people with the explicit intention of sharing it, and encouraging remixing or derivative works. There really isn't much of an online community for this when it comes to video. The closest you get is stock footage, or the occasional Free Culture project like something made by Blender.