I recently had the chance to test out a set of Dexter Industries Thermal Sensors produced for the NXT.
For those that are not aware, Dexter Industries has entered the market of producing LEGO Mindstorms NXT-compatible sensors and has come up with some pretty cool ideas such as the dSwitch (allows you to switch in-home devices on/off using the NXT) and some cool new ideas such as solar charging for the NXT as well as a flex sensor (stay tuned for more on this).