Sites

The Hypertext d20 SRD

The Sovelior-Sage System Reference Document 3.5

The Official WotC SRD

Concord Gamer's Group

The Known Lands Online Campaign Sourcebook - A well-presented campaign website.

PBWiki's home wiki

ENWorld - the biggest d20 website.

RPGNow - The best place to buy pdf products.

Drivethru RPG The best place to buy print products in pdf format.

Frilond Campaign A good site with some good house rules, and an interesting collection of new/updated/revised monsters.

Worldbuilding Advice on world-building; directed towards fantasy authors, but applicable to game settings as well.

Tools

Creature Generator - Generates advanced and/or templated creatures from the SRD

d20 NPC Wiki - Find a statted NPC or monster, or share your own.

Recommended Reading

Reference & Mythological

Arthurian Romances - Chretien de Troyes, translated by D.D.R. Owen

The Mabinogion - Translated by Gwyn Jones and Thomas Jones

Norse Mythology: A Guide to the Gods, Heros, Rituals, and Beliefs - John Lindow

Medieval Folklore: A Guide to Myths, Legends, Tales, Beliefs, and Customs - Carl Lindahl, John McNamara, John Lindow

Fiction

Mythago Wood - Robert Holdstock

Lavondyss - Robert Holdstock

The Summer Tree (The Fionavar Tapestry Book 1) - Guy Gavriel Kay

The Wandering Fire (The Fionavar Tapestry Book 2) - Guy Gavriel Kay

The Darkest Road (The Fionavar Tapestry Book 3) - Guy Gavriel Kay

Tigana - Guy Gavriel Kay

Bridge of Birds - Barry Hughart