I've been using this script for some months.
It's the best I've used for this. It's simple as it should be (just a 'Download' button) and offers all video formats available.
It was broken for a few days because of an YT update but nonetheless thank you very much to Jeppe for making it available.
This works surprisingly well. The button blends well with YouTube and gives you options to download the available sizes. I'm using Firefox 3.6.17 with no problems. Works better than the add-ons at Mozilla.