Rec.models.rockets newsgroup. Avoid the internal politics and you'll find a wealth
of useful information, and a lot of helpful people.

Rocketry Online website. A fine jumping-off point for all sorts of sub-catagories.
In particular, here you'll find a nice list of online vendors and manufacturers.

National Association of Rocketry. The governing body of model rocketry, these
are the people that keep model rocketry legal, certfify rocket motors, and publish
a lot of technical information. Here is where you'll also find all the official definitions
that separate model rocketry from other less-safe activities.

I've entered a couple of the online design contest for model rockets (even won a
(6th place) prize once.) The entries should still be online at:

Star-roc - a kid-friendly model designed based on "stars."

Micro-Trident - a downscaled version of a famous Estes model, with an interesting recovery mechanism.

BillW's Super Magnetic Fin Alignment Jig

Budget-rockets Some advice, suggestions, and sources if you're interested in building model rockets at a fraction of the price of big-name kits.

strobes.zip One of the other projects I've been working on is "recycling" disposable camera flash attachments into repeating strobes for night launches or other applications. This mostly means developing new circuit boards whose size and shape is better suited to a model rocket payload bay, and is complicated because there seem to be several versions of the components used (even in cameras from the same manufacturer!) There's probably a lot of documentation left to do, but schematics and board layouts based on the three major types of flash I've seen inside Kodak Max disposable cameras are in here:
strobes.zip
(these are in Cadsoft EAGLE format. Eagle is a moderately nice schematic and board layout tool whose manufacturer has graciously provided both cheap and freeware versions for hobbyists and non-commercial use. You can download it from here: CadSoft

Amateur Rocketry, on the other hand, is a quasi-legal (if you're very careful) rather
more dangerous activity that is actively presued by probably a couple dozen people across
the country. A bunch more (such as myself) participate in a rather more "arm-chair"
sort of way. In general, the AR crowd is defined as people who want to make their
own motors for rockets (rather than using the safer, regulated, pre-manufactured versions
used in model rocketry.) This actually encompasses a wide variety of interests ranging
from fireworks enthusiasts looking for interesting effects in rockets, to people making
an honest attempt at lowering the cost barriers involved in spaceflight.
A reasonable jumping-off point is the AROCKET mailing list web site.