WikiMoon
It's like Wikipedia, except for Sailor Moon :)
General
moonkitty.net
Moonkitty has great episode guides and an extensive merchandise section.
Three-Lights.net
Three Lights has an extensive gallery (anime + PGSM) and media/music collection.
Anilinks - Sailor Moon
Links to old, nostalgic Sailor Moon fansites / shrines. Surprisingly, many links still work.
Ai-no-Senshi.net
Really great media resources, including screencaps, icons, sprites, and other media.