I just tested Kickyoutube.com. They have a bookmarklet. I know that the mp4 download option works
FF had a plugin called Fast YouTube Downloader that I really liked but recently stopped working with the new YouTube format. Hopefully they'll fix it as it was my favorite.
My preference has been SnapzProX from Ambrosia. It will capture any screen content that appears on your Mac, video or still image. Doesn't matter what the source is - if it plays on your screen, it's easily recorded. It has been very, very useful to me. It saves the files in .mov format, which I can then transcode via Quicktime Pro to a variety of export video formats.
Get the Jdownloader. You just copy the links into it and it finds the videos. Then you add what you want to your download list. It works with a ton of other file sites as well. It is the greatest downloading program ever.
I've been using the full version for a couple of years now. It's been a great tool and has never let me down. Great for things like Flash, that have no "save target as".