Yakuake is a very cool Quake-like drop-down terminal emulator for KDE. If you've never seen Yakuake in action before, check out the following video which I've just recorded with Yakuake 2.9.8 in KDE SC 4.6:
A new Yakuake version was released yesterday (2.9.8) which finally brings support for KWin for handling its window animation (via OpenGL). The new version also comes with keyboard shortcuts to grow terminals in any diretcion and minor bug fixes.
If you're using Ubuntu (Kubuntu) 10.10 and you have installed the latest KDE 4.6, you can install Yakuake 2.9.8 (only works with KDE 4.6+) from a PPA using the following commands:
sudo add-apt-repository ppa:msb/ppa sudo apt-get update sudo apt-get install yakuake
If you're not using KDE, you can use Guake instead of Yakuake. If you want split terminals (and other features) too, check out this how-to: How To Get A Quake Like Terminal Under Gnome Using Terminator.