What are all the different accessories that I can get for an NXT?
Power
9798 Rechargeable Battery

Connectivity
LEGO 8528 Conversion Cable
Supports connecting sensors and motors from older LEGO® MINDSTORMS® RIS 2.0 (#3804) to new MINDSTORMS NXT Intelligent Brick.
LEGO 8529 Connector Cables
Enables connection of sensors and motors to your LEGO® MINDSTORMS® NXT.
LEGO 9847 Bluetooth Dongle
Allows remote control of the NXT instead of using cable.
Sensors
MS1048 RF ID Sensor
Manufactured under licence by Codatex. Reads 125 kHZ transponders of type EM4102. Various additional transponders are also available.

HiTechnic NAA1030 Angle Sensor
Manufactured under licence by HiTechnic. Measures axle rotation position and rotation speed.

HiTechnic NAC1040 Acceleration / Tilt Sensor
Manufactured under licence by HiTechnic. Measures both acceleration and tilt in three axes.

HiTechnic NBR1036 Barometric Sensor
Manufactured under licence by HiTechnic. Able to read both atmospheric pressure and temperature. Can be used as part of a weather station or to measure altitude changes.

HiTechnic NCO1038 Colour Sensor
Manufactured under licence by HiTechnic. Detects an extended range of more than 15 target colors.

HiTechnic NEO1048 EOPD
Manufactured under licence by HiTechnic. The EOPD (Electro Optical Proximity Detector) uses an internal light source to detect the presence of a target or determine changes in distance to a target.

HiTechnic NGY1044 Gyro Sensor
Manufactured under licence by HiTechnic. Accurately measures direction and amount of rotation.

HiTechnic NIL1046 IRLink Sensor
Manufactured under licence by HiTechnic. Uses Infrared signals to communicate with trains, Power Functions Motor controller and the Mindstorms RCX.

HiTechnic NIR1032 IRReceiver Sensor
Decodes commands from a Power Functions Remote Control.

HiTechnic MS1034 Compass Sensor
Manufactured under licence by HiTechnic. Able to measure the earth's magnetic field to determine direction in which it is facing. Has a built-in calibration to help reduce magnetic interference from other sources.

HiTechnic NMS1035 Magnetic Sensor
Manufactured under licence by HiTechnic. Detects magnetic fields that are present around the front of the sensor in a vertical orientation.

HiTechnic NSK1042 IRSeeker V2
Manufactured under licence by HiTechnic. Detects infrared signals from sources such as the HiTechnic IRBall soccer ball, infrared remote controls and sunlight.

LEGO 9846 Ultrasonic Sensor

LEGO 9845 Sound Sensor
Can measure sound pressure levels up to 90 dB.

LEGO 9844 Light Sensor (Output: can also be used for as Lamp)
Can distinguish between light and dark

LEGO 9843 Touch Sensor
Detects when it is being pressed by something and when it is released again.

LEGO 9749 Temparature Sensor
Can measure both temparature -20°C to +120°.
LEGO 9694 Color Sensor
Acts as a Colour Sensor distinguishing between six colours; as a Light Sensor detecting light intensities, both reflected light and ambient light; and as a Colour Lamp, emitting red, green or blue light.
LEGO 9799 Vernier NXT Sensor Adaptor
Allows you to integrate Vernier sensors with the Intelligent NXT brick and the NXT Software.

Motors
9842 Interactiv Servo Motor
Can also be used as a rotation sensor.