[pessimizer]

I use CapsLock; lowest usefulness to size ratio possible.

[NextHendrix]

Caps lock is much more useful as a home row ctrl key

[mypalmike]

Why stop there though? Mine is configured to be ESC when tapped, CTRL when held down. Took a while to get used to, but it's a big improvement compared to other ways of handling ESC on a 60% keyboard.

[JoshTriplett]

I find it most useful as a Super key, so that I can attach all my custom keybindings and shortcuts to it.