To date, only one probe has ventured to the Neptunian system, Voyager 2. Neptune was the first planet discovered with the use of mathematical equations instead of chance observation (making it my favorite planet outside of our own). The large gas giant (sometimes, Neptune is also called an "ice giant") on the outskirts of the solar system was relatively unknown until Voyager flew past in August of 1989.
During its brief visit, Voyager 2 managed to teach us a lot about Neptune and his system. Voyager found five moons, a ring system, a "Great Dark Spot" similar to Jupiter's famous Great Red Spot, and nitrogen ice volcano's on Tritan, which also turned out to be the coldest planetary body currently known to us (the black smudges you see on Triton's surface is residue from these volcanoes).