Michael Gasperi has a long history of custom (and useful) hacked sensors for both the RCX and the NXT. He recently updated his page with an NXT version of a galvanic skin response sensor. Typically this gets labeled as a "lie detector", but it's actually a lot more useful then that (I mean, how often does somebody you think is lying let you wire them up to some custom device?). Take a look, and let us know if you build something with it... or what the results are:
Truthfully, I think the neatest use for this would be simple biofeedback. Imagine something as simple as the NXT "singing" a tone that corresponds to the sensors value, and then trying to control the sound by controlling your mental state. This looks like a sensor I really need to build.