There are lot of software available on the net to download youtube videos. But the simplest and the online way is here :)
Enter the URL to the video that you want to download into the box given and press submit. It gives you the download link in return. As simple as that. No need of third party softwares. No need to register in that site. Works fine. :) Downloaded file will be in FLV format.
Note: Beware of the adult ads present in that site.