Over at Eindhoven University of Technology, students of the Master course, LEGO Beyond Toys, came up with various amazing third party products that greatly increases the capabilities of the NXT.

Water Pump

GPS Sensor

Wireless Sensor Bridge

Optical Mouse Sensor

Magnetic Grabber

Check out the details @ http://www.bartneck.de/2009/02/16/results-lego-beyond-toys-2/