On August 25, 1989, Voyager 2 passed by Neptune, giving us the first pictures of this beauty, up close and personal. Before then, all we had were pictures of a dot in the sky. In fact, before the 1800s, Neptune was thought to be another star. It was an exciting time of unexpected discoveries about the outer planets. We learned that Neptune had active storms (including the Great Dark Spot) and that its cold moon Triton had active geysers. Here is a picture of scientists at JPL looking at the new pictures coming in.
See here for memories from that day: http://www.jpl.nasa.gov/news/news.php?release=2014-288
Yesterday, (on the 25th anniversary of the Neptune flyby) the New Horizons spacecraft passed Neptune again, on its way to explore Pluto and the Kuiper Belt. Like the Voyager missions, New Horizons will keep pushing our knowledge of the farther edges of our solar system.