Birthday on Mars – the two Mars MER-Rover, equipped with six KETEK alpha particle detectors and a customized SDD, are still active and could celebrate their 5th birthday after landing on Mars on the 4th of January in 2004.
Read more about it:

http://www.jpl.nasa.gov/podcast/mer20090112.cfm
http://www.cnn.com/2009/TECH/01/03/mars.rovers.five.years/