Mediadownloader is an opensource software that lets you search, watch and download items with Google Image and YouTube. It grabs video streams while browsing with firefox (only linux). Videos can be converted automatically using ffmpeg.

There are deb packages provide in its homepage for Ubuntu installation:

You may get a dependence error while installing, for me it was missing libqt4-webkit and can be easily fixed by apt-get install command:

sudo apt-get install libqt4-webkit

After installation, found it under Applications -> Internet menu.