What are all the third party (unofficial) accessories I can get for an NXT
There are MANY companies that offer third party stuff for NXT:


Mindsensors, already mentionned in pcantin's answer


Vision Subsystem - Capable of tracking up to 8 coloured objects
Touch Panel
Numeric Pad
IR Obstacle Detector
Sony PS2 Controller Interface
Line Sensor Array
Accelerometer
Realtime Clock
Power Meter
IR Distance Sensors (Long/mid/short range)
Multiplexers for motors and sensors
Linear Actuators

HiTechnic


LEGO Certified Products, listed in the official answer.

Dexter Industries


Wifi Sensor - a wifi connection for the NXT
Inertial Motion Sensor - Acceleration, tilt and angular velocity
Thermal IR Sensor - Reads surface temperatures
Digi XBee Radio - Long distance radio communications
GPS
Solar panels

Actuonix Motion Devices Inc.


Linear Actuators

Rotacaster


Omni Directional Wheels

Microinfinity


Inertial Motion Sensor - Acceleration, tilt, angular velocity and tracks relative position

Codatex


RFID Sensor and transponders 

Nullspace


IO Expansion

Vernier


Sensor Adapter - Allows access to over 30 Vernier sensors

Generation Robots


POB-Bridge - Allows access to the POB sensors and accessories
Reseller of other sensors mentioned above

Trossen Robotics


Reseller of other sensors mentioned above



... and I'm pretty sure I forget several!