Back in early 2004, NASA sent two rovers to Mars. The rovers were named Spirit and Opportunity.

As of today, the 3rd January 2009, Spirit has now been on the surface of Mars for 5 years. This is especially impressive as the original mission was only meant to consist of 90 days on the Martian surface.

Since landing on Mars, the two rovers have collectively sent back over 250,000 images to earth and travelled over 21 kilometres.

Opportunity landed on Mars about 3 weeks after Spirit.

Information: http://en.wikipedia.org/wiki/MER-A