This is a Mud Fiddler Crab (Uca pugnax) photographed during low tide in Wellfleet, MA, on 20 September 2012. Males have one small claw and one much larger claw. This is called extreme cheliped asymmetry (cheliped is the scientific name for a claw).
The smaller claw is used for transferring mud to the mouthparts where they will scrape organic detritus off of the mud for food. The large claw is used for displaying to females.
In contrast, females have two small claws.
In both males and females, note the tall, elongated eyestalks. In some of the photos you may notice irregular compacted balls of mud; these are created when the crabs excavate their burrows. You may also spot a few burrow entrances.
Here's a view of the salt marsh where these photographs were taken.