HiTechnic Products has been making custom sensors and accessories for the original RCX for years. Now they’ve expanded their product line to support the new NXT as well, and have announced two NXT-ready products: a compass sensor and a color sensor (latter pictured above).
Here’s some additional info on the compass sensor from the HiTechnic website:
HiTechnic announces its first NXT compatible sensor, the HiTechnic Digital Compass Sensor which operates with the new LEGO® MINDSTORMS NXT system. The Digital Compass Sensor connects to an NXT sensor port and is capable of operating in full digital mode. The Digital Compass Sensor outputs a magnetic heading from 0° to 359° and with an internal accuracy of better than 1°. Heading values are rounded to the nearest 1° assuring excellent navigational characteristics.