There are two, actually, at the moment. (In the past, there was also the Sojourner, but it broke down a long time ago.) Spirit and Opportunity have been on Mars since 2004, and they were designed, built, and are currently operated by the Jet Propulsion Laboratory (part of NASA), based in Pasadena.

The JPL does a lot of the robotics stuff - some of their current ones include the Cassini mission to Saturn, the Dawn mission to Ceres and Vesta, the Mars Odyssey Orbiter and the Mars Reconnaissance Orbiter, the Phoenix lander (also on Mars - they really like their Mars!), and they keep an eye on the Voyagers, now the furthest manmade objects from Earth. Basically, if it's robotic and by NASA, JPL has probably been involved.

There are two, actually, at the moment. (In the past, there was also the Sojourner, but it broke down a long time ago.) Spirit and Opportunity have been on Mars since 2004, and they were designed, built, and are currently operated by the Jet Propulsion Laboratory (part of NASA), based in Pasadena.

There are two, actually, at the moment. (In the past, there was also the Sojourner, but it broke down a long time ago.) Spirit and Opportunity have been on Mars since 2004, and they were designed, built, and are currently operated by the Jet Propulsion Laboratory (part of NASA), based in Pasadena.