Today's Mars Photo is of Phobos, one of the Martian Moons. Phobos is 27 km at its longest point and orbits Mars at about 3000 miles above the surface in a little over 7 hours.
This image was taken by the Mars Reconnaissance Orbiter on March 23 2008. The surface is covered with craters, but the largest one, Stickney crater, can be seen on the right side of this image.
This image links to an image of Phobos' shadow passing over Mars, as taken by the Viking 1 Orbiter.