All Free.
Just go for
www.savevid.com lah. You don't hv to install any extension, addon, whatever.
If you're using FF, then go for downloadhelper addon (but note it cannot download rtmp streams, but if you're just interested in YT and other sites like Dailymotion, LiveLeak, etc, it should do fine)
If you're using Chrome, you need an extension to do it. Chrome webstore does not allow extensions that download YT videos as it's against its TOS, and YT is owned by Google too. But there's an extension that can do it but it needs some additional steps to install, so unless you're really interested, I'll skip it here.
If you're looking for multiple downloads and browser independent, then atube catcher is as fool-proof as you need. Of course there're other solutions, like IDM (Internet Download Mgr) but it costs you and you've to get its cracked version, which is not for everyone, as you need to know how to install the crack, which if you're not the least IT literate, then better not try.
