DGary – points to the updated LEGO NXT developer page –
Software Developer Kit (SDK)
Includes the NXT driver interface specification and necessary tools for creating third-party programming environments:
Includes documentation for interfacing with the MINDSTORMS NXT driver on the PC or Mac.
Executable File and Bytecode Reference
Includes documentation for the executable file format on the NXT. This document also describes how the NXT firmware’s virtual machine (VM) executes the files.
Hardware Developer Kit (HDK)
Includes documentation and schematics for the NXT and related sensors. The documentation enables you to design and develop your own sensors and actuators that can interact with and control the NXT through the various digital and analog interfaces.
Bluetooth Developer Kit (BDK)
Documents the communications protocols to the intelligent NXT brick, enabling you to create applications for various Bluetooth devices to communicate with MINDSTORMS NXT robots.
LEGO Mindstorms NXT developer page – Link.