Development resources

Map File Format
Blocks, items and data values are common to all map types.

Map Formats

 * Classic Level Format used by the old Minecraft Classic: Survival, Creative, and Multiplayer.
 * Indev Level Format used by Minecraft Indev
 * Alpha Level Format used by Minecraft Alpha (previously Infdev).

Notch's notes on NBT format files and the old Indev format.

Schematic files based on NBT format are used by a few 3rd party utilities to store sections of a map for later use.

Libraries

 * Java Classes to Deserialize Maps
 * Java Class to read and write NBT structures

Server Protocol
Classic Server Protocol used by the Minecraft Classic Creative Multiplayer server.

For info on the protocol for Minecraft Alpha, see the TkTech wiki page as well as the authentication protocol here.