Guake is a top-down terminal for Gnome, in the style of Yakuake for KDE, Tilda or the terminal used in Quake.
Guake’s properties window supports configuring keyboard shortcuts, appearences, main window height, etc. Unfortunately, the width is fixed and maximal.
Guake is available in Ubuntu universe repository. Install it via Ubuntu Software Center, or:
sudo apt-get install quake
Here are screenshots of Yakuake and Tilda (both quake style and available in ubuntu’s default repository):