The Dragonsfoot Forums
The premier spot online for the OOP versions of D&D. Regular contributors include some of the founding fathers of the game: Gary Gygax, Frank Mentzer, Steve Marsh and Jim Ward.

The Vaults of Pandius
The official site for all things Mystara.

The World of Mystara Players' Guide
This is an outstanding compilation of Mystara information geared towards players instead of DMs. It includes information on most of the Known World nations as well as character creation tips and other goodies.

Classic D&D
A site run by Harmyn of Dragonsfoot containing a lot of information about the game. It also hosts Cab's list of spells - a great collection of new spells for Classic D&D.

The OD&D Guild
A site dedicated to discussing Classic Dungeons and Dragons, original D&D and the various versions of Basic D&D. Also for fans of OD&Dities ezine.

The Acaeum
A site which details the publishing history of most of the "old school" Dungeons & Dragons products. It also lists current prices from eBay and other online sources.

Paizo.com
This is a store which legally offers Classic D&D materials in pdf format. If you are looking for a Rules Cyclopedia, it is only $4!