Mood rings were popular when I was a child. The gem stone changes color in response to "mood" which is reflected by body temperature.
I heard a story on the radio (11.29.07 entry) this morning that talked about arousal responsive tattoos. Oh my. The idea is that the tattoo is only visible in response to arousal or touch. The company promoting the concept has a number of pictures and a video (scroll to bottom of linked page) demonstration of how the tattoo responds to intimate touch. Fascinating and somewhat creepy.