NASA has some Model Rocketry pages with a lot of useful information for people new to the sport.
The rec.models.rockets newsgroup provides a lively forum for discussion of model and high-power rocketry. (Note: Your browser must be news-group-capable to follow this link.)
Associated with rec.models.rockets is an online archive . In particular, the RMRFAQ directory contains useful answers to many Frequently Asked Questions. The same FAQ information is also available as a collection of web pages.
The SEDS page at the University of Arizona includes pointers to several other good sites.
The 
Rocketry Online site is a fantastic resource, with lots of information and links to other sites. Don't miss this one!
Rocket Reviews Sort of a user written newspaper on rocketry.
Friends of Amatuer Rocketry, Inc. An experimental rocketry site in Mojave California.