Curiosity, a rover that's exploring Mars as of 2014, has a top ground speed of 1.5 inches per second, or about 0.1 mph. Curiosity landed on Mars on Aug. 5, 2012.
The first rover, Sojourner, landed on Mars in 1997. The second and third rovers, Spirit and Opportunity, were launched in 2003. All of the rovers travel very slowly so that they can take pictures and collect samples as they move. The rovers must also constantly assess the terrain around them to ensure that they do not travel in areas where they may become stuck. The Spirit rover became stuck in sand in 2009, and its mission ended in 2011. As of 2014, the Opportunity and Curiosity rovers are still exploring Mars.