A dune in the northern polar region of Mars shows significant changes between two images taken on June 25, 2008 and May 21, 2010 by NASA's Mars Reconnaissance Orbiter. This motion includes landslides and sand advancing at the dune front (upper left); changes in the position of the rest of the dune boundary relative to the fixed, underlying terrain; and changes in the position of ripples on the dune surface.
This is one of several sites where the orbiter has observed shifting sand dunes and ripples. Previously, scientists thought sand on Mars was mostly immobile. It took the mission's High Resolution Imaging Science Experiment (HiRISE) to take sharp enough images to finally see the movement.
While dust is easily blown around the Red Planet, its thin atmosphere means that strong winds are required to move grains of sand.
Photo credit: NASA/JPL-Caltech/University of Arizona/JHUAPL
Note: For an animated GIF file that shows the movement of the sand dunes over time, see here. I am not sure exactly where this image is located other than the very generic "northern polar region," so I am not attempting to link to any specific location.