Peeron – Very useful LEGO set inventory database. Also includes pictures of nearly every single LEGO part, and also a LEGO color chart.
Bricklink – A venue for individuals and businesses from all around the world to buy and sell new, and used LEGO through fixed price and auction services.
Brickset – A nearly-complete LEGO set database.
LDraw – A CAD system designed by and for LEGO enthusiasts.
LUGNET – Global community of LEGO enthusiasts. LUGNET unites LEGO fans worldwide through discussion groups, web pages, and services. Home to thousands of LEGO fans of all ages.
BrickArms – An online store where you can buy custom weapons, weapons packs, and custom minifigs. Features a wide selection of firearms and other war accessories that fit perfectly with LEGO minifigs.