Mindstorms kit is well prepared for extensions. Additional elements are sold by LEGO and other companies.
You can also prepare extensions by yourself.

LEGO publiced the whole documentation and source code of NXT Brick operation system.

• LEGO Power Functions

Rich source of knowledge about Mindstorms kit extentions: LEGO Power Functions

Available elements:

• Compass sensor,
• Color sensor,
• Accelerometer sensor,
• Infrared seeker,
• Gyroscopic sensor,
• Infrared link sensor,
• LEGO Camera.

• NXT Compass
• communication: NXT adapter RCX
• NXT PlayStation 2 interface
• NXT motor multiplexer
• NXT sensor kit
• NXT distance sensor
• NXT barrier detector
• NXT clock
• NXT pressure sensor

NXT Prototype Board is a kit to build own sensors for NXT Mindstorms. It can be used as interface for connection between NXT and other devices.

• http://www.hitechnic.com/index.html?lang=en-us&target=d20.html
• http://nxtasy.org/2006/10/29/new-products-from-hitechnic/
• http://www.electrokits.com/components/HiTechnic_NXT_Circuit_Boards

• Book: The Unofficial Guide to LEGO MINDSTORMS Robots.

• LEGO IR Protocol in C++
• LEGO Power Functions RC protocol (PDF)

Go to: lego byte code

• Detailed technical information
• http://www.extremenxt.com/lego.htm
• http://www.mustrum.de/lego/lego_engl.html