stockfootageforfree.com has several clips... and there are a few occasional updates. They're basically a front for VideoBlocks.com, from which you can get a 7 day free "spree" from their 50,000+ clip library. Afterwards, they're for $s.
BottledVideo.com used to have free HD clips... not anymore... but I think you can still get all their SD clips for free.
Archive.org has a LOT of old copyright free video (from old movies, to ads, to contributions from users.) Just read the fine print for usage.
FootageIsland (http://www.youtube.com/user/footageisland?feature=g-high-u) has a lot of HD green screen clips available, and they're still free (and good resources for practicing your green-screen "chops".)
XStockVideo.com has 960x540 versions of their clips for free.
There's a "group" on Vimeo for Free HD stock footage (https://vimeo.com/groups/freehd) -- you'll need to be a member and join the group, I think.
VidiotsChannel.com has special fx footage and some stock footage
And HollywoodCameraWork.us has some more greenscreen and "tracking plates" fun stuff to play with.
I have a couple of clips available now, but I plan to be adding considerably more on my youtube channel: http://www.youtube.com/playlist?list=PLA40CF176604014BC&feature=view_all
HTH