vixy.net will convert .flv (flash video files which is what youtube uses) to many formats that can play on your mac.

It can do:

AVI for windows (Divx + mp3)
MOV for mac (Mpeg4 + mp3)
MP4 for ipod / psp (Mpeg4 + aac)
3GP for moble devices (Mpeg4 + aac)
MP3 for audio only

Honestly the downloadable beta works much better.