I used to download videos from YouTube and similar sites using the plugin provided by Real Player 11, but I couldn’t use it on Linux, and so, I searched for another tool, and what a surprise :), I found a very nice one that’s availabe as a Firefox add-on, and it works like a charm:

https://addons.mozilla.org/en-US/firefox/addon/3006

Enjoy downloading your favorite videos :), now I feel really tired and need a nap, see you 🙂