While browsing the Internet, I came across an interesting story titled Tube2Keep - Easily Download YouTube Videos And YouTube Movies!
On visiting Tube2Keep, the site does say that it allows easy video download from your favorite sites -- such as YouTube, Google Video, DailyMotion, CollegeHumor, Break.com, Blip.tv, RedTube, etc., using this site's bookmarklet.
Now, I haven't tried this out, largely because I am not such a big fan of online videos, unless those are really of interest. Am more happy if I can instead watch the same video on a TV screen!
Surely, you should try this out, given that YouTube has a high ranking of 3 on Alexa, which means, lots of folks visit that site. Therefore, lots of friends out there may be interested in downloading videos. Enjoy!