If you're familiar with safety spoons (spoons with heat sensitivity properties that turn white if the temperature is too high), you might be intrigued by Hansacanyon -- a line of color-altering, heat sensing faucets that do what the safety spoon does, albeit via an entirely different technology.
Hansa's line of faucets for sinks, baths and showers uses embedded LEDs and presumably some kind of internal thermometer to indicate water temperature. The magenta glow offers a visual cue that the water's too hot before you scald your skin.
Aside from the fun factor, there's the safety component and, as with the spoons, this is very useful for households with kids.
See Hansa's website for more information.
(Edited from a post originally published 8.31.06)