It's not everyday that a new moon is discovered, but there's one man who seems to have quite the knack for it. Mark Showalter, a researcher at the SETI Institute, has just spotted a new moon orbiting Neptune by searching through thousands of archival images from the last decade. He can add this discovery, tentatively named S/2004 N1, to his previous collection of five moons and three rings discovered. Pretty good work!
It's actually not that uncommon for a moon to finally be noticed years (or decades!) after it is first imaged. Against a field of stars, a tiny moon is often one of the dimmest things in an image. Without a bunch of consecutive images to show the moon moving, it has no reason to stand out. In this case, Showalter found S/2004 N1 (which means the first Neptune-orbiting moon first imaged in 2004) in a series of 150 Hubble images spanning five years.
This is almost certainly not the last moon in the Neptune system, but it might be the last one we can discover from Earth. When orbiters reached Jupiter and Saturn, they found a host of small moons that were just too faint to see from Earth. When might such a probe arrive at Neptune? None are planned, so it is likely decades from happening.