To deactivate the irksome Caps Lock key on X.org, XFree and presumably other X11 servers, the xmodmap utility is used to remove the Caps_Lock keysym from the lock modifier class:
xmodmap -e "remove lock = Caps_Lock"
The xmodmap utility unbinds the Caps Lock key from the lock modifier class temporarily, thus it needs to be executed every time you restart your X session.
To save the changes permanently as the global default, append the xmodmap expression to your global Xmodmap file (usually /etc/X11/Xmodmap). To save them only for your own user, append the expression to your ~/.xmodmaprc file.

How to reconfigure the Caps Lock key on X.org/XFree to do [insert action here]?

There are several actions people would like the Caps Lock key to perform instead of doing the Caps Lock, suggested are (among others) another Ctrl key (Happy Hacker layout) or a second Backspace or Enter key. We assume you already unbound your Caps Lock key from the lock modifier class as described above ("How to deactivate the Caps Lock key on X.org/XFree?").
The key can easily be transformed to another Ctrl key by adding it to the control modifier class: xmodmap -e "add control = Caps_Lock"
To transform it to a second Enter/Return key, overwrite the Caps Lock key's keycode with the Return keysym (keycode 66 on common keyboard layouts): xmodmap -e "keycode 66 = Return"
If you want it to become a second Backspace key instead, replace the Return keysym name in the example above by BackSpace.
See also "How to deactivate the Caps Lock key on X.org/XFree?" on how to make changes permanent.