FS-300 Adjustable Bathroom Condensation Control, by DewStop
Picture a wall switch that automatically turns on the vent fan when certain people "forget" that they just took a shower—then shuts it off once the humidity level is back to normal. Now picture it tracking humidity levels to know what's normal for your home so that it won't be fooled by weather. Pretty smart.
Red this issue of This Old House online
See DewStop on the This Old House website
See this The TOH Top 100: Best New Home Products 2014 on the This Old House website