The Mars Desert Research Station (MDRS) is operated (and owned) by the Mars Society and located in the desert of Eastern Utah, on the San Rafael Swell, not too far from Moab and the Arches National Parc.
Small crew of typically six people join the habitat (hab) for short missions (usually 2 weeks) from December to May, each year since the early 2000s. At the end of this season (2013-2014) the MDRS will welcome its 141st crew. Crews are selected by the Mars Society and perform research experiments while simulating a mission on Mars. This means any activity outside the hab needs to be performed wearing a spacesuit and a hand-held radio, internet is limited (no delay in communication is set though), water is also very limited and most of the food is freeze-dried.
The base is also equipped of a greenhouse (GreenHab), an observatory and ATVs.
You can find more information about the MDRS here.