This is the easiest way one can download videos from YouTube.com. No software is required to do this.
1. Watch your favorite video on firefox till end.
2. Minimize the firefox window (Do not close it).
3. Go to /tmp folder (File System --->> tmp).
4. You can see a file name starting with Flash...
5. Copy this file and paste it in your favorite location.
Ideally it should work for all video websites. But i have not tried.