Gullies in wall of Hale Crater. Credit: NASA/MROOut of about 40 robotic missions launched toward Mars since the early 1960's, about 17 of them have been successful (I say "about" to hedge my bets, because the "success" of some of those missions is a bit gray), and of all of those, three orbiters, two rovers, and maybe—MAYbe—one lander are still active.
Here's a quick status on the active ones:
Mars Reconnaissance Orbiter: The most recent arrival at Mars (2006), MRO's 5-year mission (hmmm—sound vaguely familiar?) is to study the history of water and climate on Mars, as well as to serve as a telecommunications relay for other spacecraft. Armed with a suite of powerful instruments to study the atmosphere, surface, and subsurface of Mars—including a camera, HiRISE, that can almost read the license plates on Martian automobiles—MRO has to date sent back more data than all other Mars missions combined. It's not been a glitchless flight, however: in 2009, MRO's computer reset itself four times for unknown reasons; the last reset, in August, was followed by a 4-month operational hiatus as Earth-side controllers performed some careful programming updates to help guard against effects of any future resets. MRO resumed operation in December.
Mars Odyssey 2001: NASA's 2001 Odyssey is still going fine after nine years—although the computer glitchiness out around Mars seems to be catching: Odyssey's computer put itself into a safe mode last November 2009 in response to a memory error. This was corrected and Odyssey has resumed doing science. Among Odyssey's major discoveries was the detection of huge expanses of water ice just under the surface of polar lowlands, and the surveying of deposits of water-related minerals in various locations around the planet.
Mars Express: Arriving at Mars in 2004, Mars Express became the European Space Agency's first mission to another planet, which was recently extended to 2012. Though the Beagle 2 lander component of the mission fell to Mars and was never heard from again, the Mars Express orbiter has sent back years of captivating images and important data, including the confirmation of methane in Mars' atmosphere (whose source is in all likelihood subsurface, and the origin of which—organic or inorganic processes—is being debated).