Microsoft XNA Compiled Format
From Just Solve the File Format Problem
XNB files are created by Microsoft XNA and distributed with many games. XNB is a general serialization format capable of representing arbitrary .NET objects, but there are common definitions for textures, sound samples, 3D models, fonts, and other game data. XNB files may use LZX compression (referred to as the Xbox XMemCompress format).
Specification
Microsoft provides XNB format documentation and sample code.
