YouTube is galore with videos, and while there are lots of ways to download them, they might not provide all the options. The best way would be to download them with your chosen format (flv, mp4, mov, hd), thus eliminating the need to convert them later.
We recommend using Youtube Video Download. It scans the YouTube page for all available downloadable formats, from iPod compatible MP4s to high definition 1080p. This script is compatible with Greasemonkey, Opera, Chrome and also IE. Get it from here.
Google Chrome users can directly install it without the need of any extension, as all Greasemonkey scripts are supported.
For Firefox users, you need to install Greasemonkey and after installation (which requires restarting your browser), you are now ready to install it.