Not quite yet ready to say "rest in peace," but the Mars Global Surveyor (MGS) spacecraft has not been heard from for over two weeks now. The most recent attempts were to image the spacecraft with the Mars Reconnaissance Orbiter (MRO) which did not succeed. The next steps are to use the Mars Odyssey spacecraft to relay messages through the Mars Exploration Rover Opportunity using the low gain antennas to see if it responds. Check out this space.com article for more details.