Guake is a fancy Quake-like terminal which lets you access it by using a hotkey (user-customizable). Unobtrusive, Guake slides down from the top of the screen, and can be used while running other desktop applications, providing you with a quick access to the console.
Guake is available from Debian, Ubuntu and Fedora repository or can be downloaded from Guake’s Official Website
Another nice alternative is ‘Tilda’