1. iMovies - A blog that features extensive search and syndication options, along with a healthy catalogue of free-to-download public domain movies across a range of genres, featuring classics, cartoons and shorts. You can download or stream the videos. http://imovies.blogspot.com/
2. Openflv - Openflv features a good selection of b-movie trashola fare which you can stream or download for the most part via the services it aggregates (such as Google video). This is a great place to get hold of such timeless classics as Reefer Madness and the 1968 delight Voyage to the Planet of Prehistoric Women. http://openflv.com/tag?t=Public+Domain
3. Internet Archive - Internet Archive is the lodestone of public domain video, and there is a huge selection of video to download or stream in multiple formats across a range of genres, from advertising and ephemera to features and old TV shows. This is a treasure trove for the would-be mash-up-meister. http://www.archive.org/details/movies
4. Wikimedia Commons - Wikimedia Commons features plentiful public domain video content, a lot of it from amateur sources. You’ll need to be patient though, the organization of the content leaves something to be desired. http://commons.wikimedia.org/wiki/Category:Video
5. NARA National Archives - The national archive, hosted at Google, have a nice collection of historical movies, including lots of great NASA footage for all of those wonder-space scenes you might want to create. http://video.google.com/nara.html
6. PDComedy - If you’re looking for comedy footage in the public domain - think everything from unintentionally humorous kitsch to cartoons and I Love Lucy - this should definitely be on your list. http://www.pdcomedy.com/
7. Stage6 - Stage6, the video sharing platform for high-quality DivX recordings, has a nice selection of public domain videos to explore. The quality is among the best you’ll find, although you may need to install DivX on your PC first to watch the movies. Buster Keaton, Betty Boop and Charlie Chaplin prevail. http://stage6.divx.com/videos/tag:public+domain/order:length
8. Public Domain Torrent - If you’re familiar with BitTorrent technology, this site has nearly a thousand movies in various formats for you to download without wondering if the MPAA are going to come to your house and flog you with a law suit. Well worth a look. http://www.publicdomaintorrents.com/
9. EMOL - EMOL has great selection of public domain video free to download across a vast range of genres. The highlight for me has to be David Hasselhoff’s delightful music video “Jump in my car” (yes, featuring the inimicable KIT). http://emol.org/movies/freemovielist.html

There are a lot more sources out there, especially if you are willing to stray into Creative Commons waters - but bear in mind that the popular Non-Commercial license won’t won’t allow you to use the footage in any commercial work - if you want to promote your business, sell your video, or even run it along web-based advertising, you are stepping into murky waters.