The Mariner series of spacecraft were the first designed to explore other planets. The Mariner missions led to a number of firsts, including the first planetary flyby (of Venus), the first planetary orbiter (of Mars), and the first gravity assist (Mariner 10 used a gravity assist from Venus to reach Mercury).
Mariner 1
Vehicle: Atlas Agena B
Launch Date: 7/22/62
Mission: Venus flyby
Outcome: A combination of hardware and software errors caused the craft to veer off course during launch. It was destroyed by the range saftey officer.
Mariner 2
Vehicle: Atlas Agena B
Launch Date: 8/27/62
Mission: Venus flyby
Outcome: Successfully approached to within 34,833 km of Venus on December 14, 1962. It was the first probe to measure the solar wind and it returned considerable atmospheric data about Venus. It is still in orbit around the Sun, but no longer functional.
Mariner 3
Vehicle: Atlas Agena D
Launch Date: 11/5/64
Mission: Mars flyby
Outcome: Its protective shroud failed to open after launch, preventing deployment of its solar cells. It never reached Mars.
Mariner 4
Vehicle: Atlas Agena D
Launch Date: 11/28/64
Mission: Mars flyby
Outcome: Successfully approached with 9,846 kilometers of Mars on July 14, 1965. It returned the first close-up pictures of Mars. It managed to survive usefully in solar orbit until December 1967.
Mariner 5
Vehicle: Atlas Agena D
Launch Date: 6/14/67
Mission: Venus flyby
Outcome: Successfully passed within 3,990 km of Venus on October 19, 1967. Returned much more data than Mariner 2. Discovered that Venus' atmosphere was mostly carbon dioxide. Operated until November 1967, it is still in solar orbit.
Mariner 6
Vehicle: Atlas Centaur
Launch Date: 2/24/69
Mission: Mars flyby
Outcome: Successfully approached within 3,430 km of Mars on July 31, 1969. It returned pictures, atmospheric measurements and a celestial mechanics experiment precisely measured the mass of the planet.
Mariner 7
Vehicle: Atlas Centaur
Launch Date: 3/27/69
Mission: Mars flyby
Outcome: Along with its sister ship, Mariner 6 (wich arrived only four days earlier), it returned 143 approach and 55 close-up pictures of Mars (covering 20 percent of the planet); ending the canal debate. Technology had improved since Mariner 4, it could transmit data 2,000 times faster.
Mariner 8
Vehicle: Atlas Centaur
Launch Date: 5/8/71
Mission: Mars orbiter
Outcome: Vehicle lost in launch failure.
Mariner 9
Vehicle: Atlas Centaur
Launch Date: 5/30/71
Mission: Mars orbiter
Outcome: Successfully orbited Mars and mapped 100 percent of the surface. Its pictures gave humans a new look at Mars: a planet of volcanos and huge canyons; with evidence of earlier flows of water. Operations ceased in November 1972, but the craft is still in orbit around Mars.
Mariner 10
Vehicle: Atlas Centaur
Launch Date: 11/3/73
Mission: Mercury flyby (multiple)
Outcome: Using a gravity assist from Venus, it flew by Mercury three times over the course of a year, approaching to within 327 km of the surface, returning pictures and other data. It was turned off on March 24, 1975, and may still be in solar orbit.
Source: http://msl.jpl.nasa.gov/Programs/mariner.html