Spirit was one of the robot rovers that landed on Mars in 2004, together with Opportunity, the other rover. Spirit was only supposed to last 3 months, however, it had long passed its lifespan and continued on.
Spirit had encountered major problems. For example, in 2006, one of its front wheels stop working; in 2008, it got stuck at the edge of Home Plate Plateau. Despite this, the rover also accomplished many major achievements. For example, in 2005, the rover took the first ever pictures of a "dust devil"* on Mars; the rover was the robot that broke the record for climbing the tallest mountain (300m) ever on another planet. It was one of the greatest interplanetary missions in history.
Spirit's mission ended due to low power generated on the solar panels to power up the rover. There was a gigantic martian winter that blocked out the sun and thus, leaving it with little or no power to function it again. The last communication was on March 22, 2010. NASA concluded the Spirit mission on May 25, 2011. :(
Now, the mission focus is on its twin rover, Opportunity, which is now on its way to Endeavour Crater, which is still another 4 kilometers. It may take the rover roughly 2 Earth Years to get there. So sit back! :)
Spirit Rover (MER-A)
4 January 2004 - 25 May 2011