This is an awesome tip courtesy of @lucianop (twitter.com/lucianop).
To download videos from youtube.com onto your computer, simply add "kiss" an the beginning of url (e.g. instead of ' http://www.youtube.com/watch?v=gjAZ5esOBZw ', make it ' http://www.kissyoutube.com/watch?v=gjAZ5esOBZw ').
The downside is that the only format available is FLV. For an option (FLV and MP4), you can also use www.savevid.com