if you dont want to download anything heres a link.

http://vixy.net/

it converts to a lot of formats.
i use it when i want to download something for my psp.

i missed iherduliekmudkip's post.
use his link. ^^

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

It works quite well and can also handle videos from sites other than youtube (My big favorite is crunchyroll)

However, with this, and most other video downloaders, you need to have an FLV player in order to be able to watch them. If you don't want to do this, then I recommend http://zamzar.com/url/ which allows you to punch in teh address of teh video you want to download, as well as the filetype you want it to be converted to. You'll then receive an email that has a link where you can download the video you requested.

If u wanna play FLV then u should download K-Lite Codec Full Pack
It works quite well