Rovers have several advantages over stationary landers: they examine more territory, they can be directed to interesting features, they can place themselves in sunny positions to weather winter months and they can advance the knowledge of how to perform very remote robotic vehicle control.
Mars 2, Prop-M rover, 1971, Mars 2 landing failed taking Prop-M with it. The Mars 2 and 3 spacecraft from the USSR had identical 4.5 kg Prop-M rovers. They were to move on skis while connected to the landers with cables.
Mars 3, Prop-M rover, 1971, lost when Mars 3 lander stopped communicating about 20 seconds after landing.
Beagle 2, Planetary Undersurface Tool, lost with Beagle 2 on deployment from Mars Express in 2003. A compressed spring mechanism was designed to allow movement across the surface at a rate of 1 cm per 5 seconds and to burrow into the ground and collect a subsurface sample in a cavity in its tip.
Spirit (MER-A), Mars Exploration Rover, launched on June 10, 2003 at 13:58:47 EDT and landed successfully on January 4, 2004. Nearly 6 years after the original mission limit, Spirit had covered a total distance of 7.73 km (4.80 mi) but its wheels became trapped in sand. Around January 26, 2010, NASA conceded defeat in its efforts to free the rover and stated that it would now function as a stationary science platform. The last communication received from the rover was on March 22, 2010, and NASA ceased attempts to re-establish communication on May 25, 2011.
Opportunity (MER-B), Mars Exploration Rover, launched on July 7, 2003 at 23:18:15 EDT and landed successfully on January 25, 2004. Opportunity surpassed the previous record for longevity of a surface mission to Mars as of May 20, 2010 and surpassed the previous record for distance traveled off-Earth as of July 28, 2014.Opportunity is still operational and mobile as of August 2014.
ExoMars, by the ESA. Planned Mars launch 2018. The rover will use stained glass to prevent UV from changing image colors, allowing for true color images of the surface of mars. 
Chinese Mars Rover, has been planned for a pre-2020 launch, as part of a sample-return mission.
One experimental design, not proposed for any actual mission, is:
Mars Tumbleweed Rover, a wind-propelled rover.
NASA rover goals
NASA distinguishes between "mission" objectives and "science" objectives. Mission objectives are related to progress in space technology and development processes. Science objectives are met by the instruments during their mission in space.
The details of rover science vary according to equipment carried. The primary goal of the Spirit and Opportunity rovers is to discover "the history of water on Mars". (The presence of usable water would greatly reduce manned mission cost.)
The four science goals of NASA's long-term Mars Exploration Program are: