[earlz]

If you're on a mobile device without a keyboard, I made an alternative version that changes on click rather than keypress:

http://jsbin.com/iPIrucU/1

[jere]

Derp. I figured someone else would beat me to it. Press or click. It goes through 3 contrasting pairs (based on the wikipedia article):

http://jere.in/opponentprocess.html