National Association of Rocketry (NAR)
The NAR is the oldest and largest hobby rocketry organization in the US. They establish safety standards, certify motors, sponsor contests and educational programs. Most importantly for many of us, they also provide a very generous insurance policy covering our activities; without it, there is very little chance that we'd get to do this.
Tripoli Rocketry Association (TRA)
Similar in purpose the the NAR, the TRA has a more experiemental focus, and is more attuned to high-power rocketry enthusiasts. In fact, the TRA certifies most of the high-power motors; the TRA and NAR have cross-certification agreements so that you can fly any certified motor at either organization's launches (subject to your Cert level, of course!) They also have an "experiemental" wing that promotes safety for hobbyist-built motors and reloads, and they sponsor experimental lauches for that purpose.
The Rocketry Forum
This is a really good board for communicating with other rocketry enthusiasts, you can share ideas, tips and tricks, and commiserate with others when things go boom.