Today's Image of Mars is a compilation of dust devil images from Mars Exploration Rover Spirit as taken in Gusev Crater. These images were taken on Sol 543 of Spirit's mission, which was on July 12 2005.
Gusev Crater is approximately 166km in diameter and is thought to have formed between three and four billion years ago. The crater was likely home to a lake in the distant Martian past, as Ma'adim Valles drains into it and the ground is rich with sediments that are up to 3000 feet thick.
Images of dust devil tracks in Gusev Crater were taken from orbit and investigated by Mars Rover Spirit. There is speculation that Spirit lasted so long on Mars because dust devils helped clean its solar panels.
Clicking this image will take you to the original, slightly larger version of this from NASA.