ClipGrab is a free downloader and converter for YouTube, Vimeo, Metacafe, Dailymotion and many other online video sites.
It converts downloaded videos to MPEG4, MP3 or other formats in just one easy step.
Full article here:
Download Videos From Youtube and Other Sites Using ClipGrab (Unixmen)
NOTE: ClipGrab is also available in Windows and Mac OS X versions.