I use Downloadhelper, it displays all the versions of the video you can download if there's more than one (flv, mp4 and so on) and you can copy the url to the clipboard if you want to add it to any download manager...
I am currently using the dl helper too.. You can find the application in the plugin section of mozilla. It's one of the best tools for this job since it displays vids not only from youtube, but also any media available on almost every site (for example: music tracks on myspace, and any streaming media)
I have this in my favorites bar and just click on it when I'm watching a youtube vid I want to download. It downloads vids in mp4.
I've found that http://file2hd.com works well for this purpose (as well as ripping videos/music from other kinds of sites), and you don't even have to install anything.
I've been using YousableTubeFix for Greasemonkey for a while now and its excellent, really does enhance youtube a good bit.