I've been using the Ant video downloader with Firefox for a few months. It works quite well with most streaming video sites incl Youtube. I do turn off the options to send downloading stats etc for privacy reasons. It also has a version for Internet Explorer but I've never tried it.

http://www.ant.com/video-downloader

It saves most videos as MP4 and occasionally as FLV both of which can be viewed with VLC Videolan player.