National Association of Rocketry - the oldest and largest spacemodeling organization in the world with over 5900 members and 165 affiliated clubs across the U.S.
Tripoli Rocketry Association - dedicated to advanced rocketry in many forms, to pushing the envelope of civilian rocketry, and to do so in a safe and scientific manner.
Aerospace Industries Association - a major TARC sponsor, it is the premier trade association representing major aerospace and defense manufacturers and suppliers in the United States.
NASA - doesn't need an explanation does it?
NASA interactive rocket simulation software - a VERY basic version of RockSim or OpenRocket.
Prairie State Rocketry Club - local club affiliated with the National Association of Rocketry and home of our very own Gary Kawabata.
Space.com - not so much a rocket site, more of an astronomy site but has some some cool stuff!