RO Information Sites
The Official Ragnarok Online Website
RateMyServer.Net - Complete Monster, Item, and Quest databases, an interactive RO worldmap, and more!
iRO Wiki - Covers almost every RO topic imaginable; all your RO questions are answered here
RODE: RO Database - Another database with calculators to help you optimize your builds
RO-Market - An RO market price tracker; no longer updated, but still useful

Character Simulators
Ragnarok Character Simulator - Flash based
Rune-Nifelheim Character Simulator - Updated character simulator with some extra perks
Character Simulator - Uses an easy web interface; not updated