I've been waiting for NASA's rover Curiosity to land safely so I can tell you that we now have Lincoln on Mars. Yes, Abraham Lincoln on the Red Planet.
The penny, a 1909 "VDB" penny, is used as a calibration target for the Mars Hand Lens Imager (MAHLI) instrument, which is to be used to take extreme close-ups of rocks and soil. The penny, provided by MAHLI's principal investigator Ken Edgettt, is a nod to geologists' tradition to place a coin as a size reference in close-up photos of rocks.
Besides the penny, the MAHLI calibration target includes color chips, a metric bar graphic, and a stair-step pattern for depth calibration.