A. E. Wallace's Posterous

Keep Tube is a web-based video downloader that handles a wide range of formats and web sites. Where Keep Tube really shines is grabbing the highest quality video from a given site, where many other video downloaders fall short.

You'll find no shortage of web sites devoted to helping you download video content. Keep Tube stands out by supporting a wide variety of sites like YouTube, Google Video, Dailymotion, Mega Video, Metacafe, Veoh, and more. Of interesting to users who don't stray too far from popular sites like YouTube and Dailymotion is Keep Tubes support for grabbing top quality and restricted files—check out this comparison chart to see how Keep Tube stacks up against other popular downloaders.

Keep Tube is a free service available as a web-based tool or a Firefox extension.

I'd imagine YouTube doesn't like you downloading videos, but since users still want to grab the occasional video to show to friends while offline, this should help.