Follow Method 1 or 2:
Method 1. (requires Firefox)
If you use Firefox, get this plugin, and install. Next time you open a video on youtube, you will get option to save it in different formats on the right sidebar.
(See Screenshot on Right.)
Method 2. Direct Download
Visit KeepVid or KeepHD and copy paste Youtube URL and snap, you get option to save it in different formats.
You can also checkout our Recommended Add-ons for Firefox.