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.
Single Time Machine Backups
Tired of Time Machine running all the time? You can turn it off in the Time Machine preference pane, but still initiate a single backup by choosing Back Up Now from Time Machine's menu bar icon. Of course, your backup is much less likely to be up to date, but Time Machine won't be taking any resources while you're trying to work.