I used to have a xmodmap configuration that made my useless capslock key work as a mod3 key, but a recent archlinux update broke it. It seemed my xmodmap configuration just got ignored, because "xmodmap -pm" seemingly gave the right output but my capslock was still working as a capslock key. After a bit of searching, I found a post on the bbs (which I can't find anymore, sorry) that explained the fix:

~/.Xmodmaprc should contain the following:

remove lock = Caps_Lock
keysym Caps_Lock = ISO_Level5_Shift
add mod3 = ISO_Level5_Shift

Then you (of course) just run it with a simple:

$ xmodmap ~/.Xmodmaprc