Best overviews: The Nine Planets Wikipedia
Best physical data and mission information: National Space Science Data Center
Best images: Hubble Space Telescope Keck II Adaptive Optics Images and Story NASA's Planetary Photojournal
Official nomenclature: USGS Gazetteer of Planetary Nomenclature
Rings information: The Neptunian Ring System at the Planetary Rings Node Voyager Neptune Science Summary
Moons information: Scott Sheppard's Neptune Satellite Page NASA's Solar System Dynamics pages: Satellite Properties and Satellite Elements
Data sources: Neptune Viewer 2.1 Voyager Image Catalog Query Form 2.0