The Mars Reconnaissance Orbiter's HiRISE camera recently revealed fascinating shots of a dune sea of sorts in a crater of the Hellas impact basin. What has officials at NASA excited about the dunes is their symmetric nature.
The dunes here are linear, thought to be due to shifting wind directions. In places, each dune is remarkably similar to adjacent dunes, including a reddish (or dust-colored) band on northeast-facing slopes. Large angular boulders litter the floor between dunes.
The most extensive linear dune fields known in the solar system are on Saturn's large moon Titan. Titan has a very different environment and composition, so at meter-scale resolution they probably are very different from Martian dunes.
Link. See more stunning images (like frosted dunes) at the HiRISE site.