A Big Mouse Cursor
I spend about ten hours a day staring at two 27-inch Apple cinema displays. It makes coding great. But, with that much screen real estate, I keep losing my mouse cursor. I'll have to jiggle it around for half a minute trying to find where it's disappeared to.
Yesterday I discovered OS X has an option in the Universal Access Preferences pane that lets you adjust the size of the cursor from normal all the way up to holy-gigantic. I have mine set to a comfortable 33% — which is just big enough to keep from getting lost, but not so large that I can't tell where I'm clicking.
I haven't misplaced my cursor since.