Do you remember Quake 3 game? It has a drop-down console which will appear when "`" key is pressed(where you can enter cheat-codes :P ). I used to find this console very cool. And now I can have similar terminal on my linux box.
Yakuake is a drop-down terminal emulator based on KDE Konsole technology. So its installation on Ubuntu requires lots of KDE libraries. So I searched for other GNOME based options.
Tilda is one gtk+ based terminal emulator which really disappointed me. This is really slow especially with animation effects enabled. Guake is also a another good option.
After trying all these options, I installed Yakuake with all KDE libraries and Konsole and it is a perfect Quake-style terminal. The default shortcut to toggle the launch of this console is "F12". You can change this shortcut to "`" key but I have already configured this key for my "screen" application.
Here is the screenshot,