The ecobutton. Its creators say its a visual reminder to power down your computer -- plug it into a USB port and it lights up, and then when you leave your desk for any extended amount of time, push it and it will send your computer into power-save mode.
Something you're probably capable of doing without a big plastic button ... But maybe something you forget to do on a regular basis?
(If you like the ecobutton, they're giving one away at The Alternative Consumer.)