If u use firefox browser there are loads of plugins for downloading video's
fast video download & ook to name but 2
Fast Video Download can save embedded video files from following video-hosting servers:
Angry Alien, Anime Episodes, Anime Forever, Bad Jojo, Blip.TV, Bo-Funk, Bolt, Break.com, Current.TV, Daily Motion, eVideo Share, Google Video, Grinvi, iFilm, Libero.IT Video, My Space, PornoTube, Put File, Streetfire, Totally Crap, You Tube, V-Social and Vidiac.
Ook? Video Ook! enables you to download embedded videos from several systems (such as YouTube, Google Video, Pornotube, Metacafe and many more).
perhaps give them a try with the site you are trying to download from.