Development resources

Map File Format
See these pages for information on Map Editing:

Map Editing (general)

Map Editing (hex)

Java Classes to Deserialize Maps

.dat Map Format

Map Format (NBT) 1 2

Java Class to read and write NBT structures

Packet Protocol
Every packet starts with a byte representing the Packet ID.

Resources
(Disclaimer: This information is currently biased towards C and Java.)