As today's edition of the XKCD comic strip reminds us, the Opportunity rover has been exploring Mars since January 25, 2004 -- way, way longer than the 90 (Martian) days it was originally designed for. The twin rover Spirit, which landed on Mars three weeks earlier, unfortunately got stuck in 2009 and ceased to communicate in 2010. But Opportunity is still at it, to the delight and amazement even of the engineers that designed it.
For more on the Opportunity rover see its Wikipedia article (link) and the NASA Jet Propulsion Laboratory pages on Mars rovers (link) and Mars rover news updates (link). See also the Explain XKCD page about the strip (link).