As users become more experienced with Linux, the almost invariably learn to love (or at least tolerate) the powerful command line interface of the terminal. For super-quick access to the terminal window, I recommend a handy utility called Tilda, which places the terminal in a dropdown menu at the top of the screen. Hit a key and it slides out from just beneath the panel. Hit a key again and it slides back up again.
The beauty of Tilda is that it's always there in the background. I can start a system update with sudo apt-get upgrade, for example, and then hide the Tilda console until it's completed. There's no risk of accidentally closing a terminal window and thereby killing any processes I started.
It also means I don't have several unused terminal windows hanging around on the desktop. Just like a standard terminal window, Ctrl+Shift+T will open new tabs in Tilda (type exit within each tab to close it).
I've tweaked Tilda's settings so the window is semi-translucent, and centred in the middle of the window. See this screenshot for an example of my settings.
Keir Thomas is the author of several books on Ubuntu, including the free-of-charge Ubuntu Pocket Guide and Reference.