Color-less Classic -- A friend at Apple notes that Color Classic users can move the contrast slider bar in the Screen Control Panel all the way to the left, making the screen go pitch black. It would seem that users in that situation are stuck, since they can't see the slider bar any more, but pushing the screen contrast button on the Color Classic's front bezel will bring the screen back up.
Option-click to Hide Apps Quickly
This trick has been available in the Mac OS for years, but many people still don't know it. If you have too many windows cluttering up your screen, you can hide specific ones easily as you work. When you're in any application, hold down the Option key and click on another app's window, on the Dock, or in the Finder to switch to that other app and simultaneously hide all the windows in the previously current app.