I saw the post below and thought and type this.
Requirements:
Mac OS X
Safari (don't know how to do it in Firefox)
Perian
Instructions:
1. Open the youtube video you want
2. Type alt+command+a Or go to Window -> Activity
3. Find the largest file item (usually around 10MB) Or
http://blablabla.youtube.com/get_video?video_id=blablablabla
4. Select it
5. Copy it (command+C Or Edit -> Copy)
6. Open the downloads window (alt+command+L Or Window -> Downloads)
7. Paste the link
8. Rename the file "getvideo" to "getvideo.flv" (or change the name too if you want)
9. Open it with quicktime (with perian already installed)
10. Export it to quicktime movie (command+E Or File -> Export)
11. Enjoy!
Extra Step:
Use YoutubeHD to make the videos higher quality
YoutubeHD Post
Direct Download
Have fun!