OK, all you Mac-heads, a bit of help please. I've managed to turn something on, and I don't know how to turn it off again. Right now, whichever control has focus is "highlighted" by a black box. Trivial, I know - especially in light of all the important stuff that I can't get working at the moment, but it's driving me mad.

I almost certainly turned it on with some clumsy miss-keystroke, but I've no idea what it might have been. I've hunted all through the System Preferences, concentrating on the Keyboard and Universal Access panels but I can't find whatever it is I've turned on. Any hints?