xkeycaps and xmodmap
Very cool programs, xkeycaps is a program written by Jamie Zawinski, the uber-hacker who created the first Netscape V1.0 for UNIX and was one of the main organizers of Mozilla it shows you your current keyboard layout, and can write a .xmodmap file.
xmodmap allows you to remap any key on your keyboard. For instance, I have remapped my numeric keypad to give the same keycodes as the arrow keys, so that I can use my numeric keypad to scroll in gnome-terminals and Netscape. I have also recently remapped my Caps Lock key to a Control key, so that it's easier to enter control codes into emacs. :)
So now, I have two Control_L keys:
keycode 0x42 = Control_L
keycode 0x25 = Control_L