The only probe to visit Neptune was Voyager 2 in August 1989. This is because Neptune is so far away that it takes about 12 years for a probe to reach Neptune. Since Neptune's orbit is so huge there are few times when it is close enough to get a probe there in a relatively short time. By the time the probe get to Neptune the technology on the probe is twelve years old.
Two different proposals were put forward in 2005 for unmanned spacecraft to explore Neptune and its moons, especially Triton. These studies are part of NASA's Vision Mission program to develop long-term space exploration goals.