Any advice on downloading from youtube?

[kickassangel]

Firstly - what I want to do is legal, I'm not trying to infringe copyright. (Just to put that out there.)

BUT - any advice on how to download something from youtube to an MP4 file?

I know that there are loads of tutorials about this on youtube, but which ones can I trust? (obviously, I trust the randoms from MN more than youtube!)

I'm using a Mac (Air) and want to drop it into iMovie. The clip is very short, less than 2 minutes.