Ctrl-Alt-Backspace

This is nothing You won't find easily with Google, but for convenience it's here, too:
By default, the Ctrl-Alt-Backspace key combo does... nothing. In order to have it kill the X server (that's what it used to do in the good ol' times), simply use the GNOME menu and go to "System"->"Preferences"->"Keyboard"->"Layout">"Options..."->"Key sequence to kill the X server" and mark the entry.
That's it! You're done (already)!

UPDATE: There is a key in the gconf database which controls this behaviour. The preferences set this key. However, changing the value of the key for the gdm user does not enable Ctrl-Alt-Backspace during the login screen.
A workaround is to just use Alt-SysRq-K (Secure Access Key) instead of Ctrl-Alt-Backspace. The effect in GDM/GNOME is the same, as the SAK combo kills every process on the active tty.