For FireFox :
http://add0n.com/youtube.html
"Video and Audio Downloader is a free open-source video downloader for Firefox. This Firefox extension downloads videos in all available formats (FLV, MP4, WebM, and 3GP) with video quality of your choice. It also contains a pure JavaScript library to extract the ORIGINAL audio file out of FLV videos. Also, it optionally downloads subtitles as well!