Instantly Darken your Mac's Monitor.
Here is a nice, quick one. If you’re working on an (ahem!) important confidential document and someone walks into the room who shouldn’t see your screen, you can instantly darken you Mac's screen by pressing ctrl-shift-eject. To turn the brightness back to where it was, press any key or move the mouse.
Source: This tip comes from a post by Matt Klein on the Small Dog Electronics Barkings Blog.