I'm a video junkie, I admit it. While i'm perfectly fine with keeping them all online, sometimes you just want to download and collect them so you can watch them all in a row or watch them by devices that aren't so compatible with the insanely large flash animations (like my MSNTV 2. Anyway, unmediated has cooked up a couple of Greasemonkey scripts that let you download Google Video and YouTube stuff. Videoriffic.

unmediated: Greasemonkeying with Google Video and YouTube