Now, there was a time when I found myself criticising Linux for an overdependance on the command prompt and too much enthusiasm to drop to it from the X GUI.
Life being what it is, I then found myself trying to log in without X, straight into console, because I was trying to install the nVidia drivers so that I could try out the 3D desktops. Try as I might I couldn’t find an option to log in in text only. I could change consoles with Ctrl+Alt+F1 etc and Ctrl+Alt+Backspace would only do something when pressed twice. And that restarted X rather than stopping it.
The answer is to switch to another console (probably with the aforementioned Ctrl+Alt+F1) and entering the following text.
su (enter root password when prompted)
init 3

I then restarted the computer once the drivers had been installed by pressing the power button on the notebook I was using  - I can never remember how I got the shutdown command to work, even after reading the help.