There are a lot of video downloaders out there, but not many support very many sites, they usually work on one, or a small handful of sites.  This one works for a TON of video sites.. 

This particular extension is only usuble in Firefox, but man does it support a lot of sites.  Check out the list if you realy want to know:

Youtube, Google Video, iFilm, Metacafe, Dailymotion, Myspace, Angry Alien, AnimeEpisodes.Net, Badjojo, Blastro, Blennus, Blip.tv, Bofunk, Bolt, Break.com, Castpost, CollegeHumor, Current TV, Dachix, Danerd, DailySixer.com, DevilDucky, Double Agent, eVideoShare, EVTV1, FindVideos, Free Video Blog, Grinvi, Grouper, Hiphopdeal, Kontraband, Lulu TV, Midis.biz, Music.com, MusicVideoCodes.info, MySpace Video Code, Newgrounds, NothingToxic, PcPlanets, Pixparty, PlsThx, Putfile, Revver, Sharkle, SmitHappens, StreetFire, That Video Site, TotallyCrap, VideoCodes4U, VideoCodesWorld, VideoCodeZone, vidiLife, Vimeo, vSocial, Yikers, ZippyVideos...

Basically, any site with embedded video files.

Simple to use, simple to install, very versatile.. what more can I say?

Check out VideoDownloader now to check it out.

Alternatively, you can go to the VideoDownloader web page and put in the link to the page with the video, and it will give you a link to the files.. if you're looking for more of a "one time use" kind of thing.