Esc <-> Caps
The original Unix terminals (or at least the one vi(1) written on) had the esc key somewhere around the tab on PC keyboards. This makes sense if you think about vi's commands.
Well, we need tab, but most of us don't use the caps key, so we can just swap them right away.
You only need to create a file with this content:
! Swap caps lock and escape remove Lock = Caps_Lock keysym Escape = Caps_Lock keysym Caps_Lock = Escape add Lock = Caps_Lock
Or you can just have them both as an Escape key.
! Making caps acting as esc remove Lock = Caps_Lock keysym Caps_Lock = Escape
You can save it to a file something like .swapesc and call it from .xinitrc: