Provided, of course, you’ve got a Mac. 
Haven’t had a chance to try this out yet, but I’ve heard good things about it. Of course, only do this with videos that aren’t impacted by copyright laws. Like this video of the Tucker I shot.
There’s a nifty Firefox plugin called VideoDownloader you can get that’ll download any video you find on a site, be it YouTube or Google or whatever 
Works on a PC too.
Maybe a tiny bit offtopic, but does anyone know of any similar tool for macromedia flash files?
Unfortunately all of the vids on YouTube are in Flash Video format (FLV). You’ll need to convert them if you want to play them anywhere but in a browser window or Flash video player.
There are several FireFox plugins that can download off of YouTube.