AstralWizard - probably the oldest and the most popular HOMM site. Strategy, polls, discriptions of spells and map locations, whishlists, listing of bugs, latest news, multiplayer league, map reviews, campain tips and more. The site is not an eyecandy, but overloaded with info.
Site takes quite long to load though.
JB's Heroes 3 Maps - HOMM3 and AB expansion maps to download. About 45 maps total to date. Maps are accompanied by little descriptions. No design but loads reasonably quick. The H3 Trio - very informative HOMM3 site with strategy, lists of artifacts, buildings, creatures, heroes, skills, spells, map locations, campaign lists, combat descriptions and over 50 maps to download.