What are all the third party (unofficial) accessories I can get for an NXT Similar to this question but I'd like to know about 3rd Party/aftermarket options instead please.

I've heard about motor multiplexers and the like but are there others, i.e. more sensitive colour detectors etc.?
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!