The variety of home automation projects using capacitive and photocell sensor notifications is expansive. Here are just a few ideas to consider pursuing:
Place an XBee+photocell-equipped Arduino, powered by a rechargeable battery pack, in your refrigerator or freezer to detect how often and how long the doors are left open. Use this data to calculate energy expended each month as a result of these encounters. If such expenditures are excessive, broadcast emails and/or tweets to cohabitants reminding them of their growing carbon footprint.
If groping for a light switch is a frustratingly frequent chore, tweak the capacitive sensor configuration to be used to turn on basement or garage lights by touching foil tacked to a wall or ...