All the other monitor mirrors on the market seem like novelty, vanity pieces in comparison to the Chimp Monitor Mirror [1] ($9.99).
While the device does include a mirror, it seems to be made for those of us who are convinced our coworkers and bosses like to sneak up on us, not just the crowd that wants to have a handy lip-gloss mirror.
It's a convex mirror (3 inches in diameter) that fits easily on either corner of your monitor with accompanying Velcro. It comes in black or white.