This is a good recommendation, but I believe it only works in Safari.
I would say this is definitely the easiest/fastest solution out there. It is a javascript and it requires no installing of anything. While watching a video simply click the on the bookmark and a download dialogue will appear. Click on it and it'll download straight to your computer in mp4 format.
I use Cosmopod. It adds a button in the Safari toolbar. You just click it and it automatically downloads whichever flash based video is currently playing.