The SRD

Basics and Ability Scores

People

Alignment and Description, Races, Character Classes, Skills, and Feats

Items

Equipment, Special Materials, and Magic Items

Combat

Combat

Deities

Deity

Magic

Special Abilities and Conditions, Magic Overview, and Spells

Monsters

Monsters, Types, Subtypes, and Special Abilities

Adventuring

Carrying, Movement, and Exploration, Treasure, Wilderness, Weather, and Environment, Traps, and Planes

Other Srds

Psionics, Epic Levels, Divine

Official D&D Worlds

Al-Qadim, Birthright, Blackmoor, Council of Wyrms, Dark Sun, Dragonlance, Eberron, Greyhawk, Forgotten Realms, Jakandor, Kingdoms of Kalamar, Lankhmar, Mystara, Pelinore, Planescape, Ravenloft, Rokugan, Spelljammer, Wilderlands of High Fantasy

Unofficial D&D Worlds

Domains of Divinity, Kerell, Teatrail Falls

Gaming Accessories

Dungeons and Dragons Miniatures, Maps & Terrain Tiles, Dice

3rd Party D&D Worlds

Áereth (aka DCC World), Dreadmire, Erde, Golarion, Ptolus