+1 for youtube-dl (link above)
it's free and it's easy
it's light as it runs on command line
I gotta throw in another +1 for youtube-dl.
Doesn't require installation.
Will keep working as long as the project isn't killed.
Will not spy on what you get. Can't say the same for web-based stuff.
You can use Tubemate of Devian Studio. See detailed information at DevianStudio.net homepage
you may want to try savefrom.net..