How about using an 8051 to monitor a water sensor in the cleanout drain so if the water level rises above some threshold (as would be the case antecedent to a toilet overflow) it could signal a valve to close (shutting off water to the house) and alert the homeowner that they need to snake their drain before they also have to change the carpet padding. And then you could add a Vacation mode to the system wherein the 8051 could monitor for water flow in the line to the house and, upon detecting the same, could again shut off the water to the house. This way people could leave their lawn sprinkler operating (the one Michael suggested) and not worry that a burst washing machine hose might flood the house. Or how about using an 8051 to monitor photosensors to detect the position of the sun along the eccliptic so it could adjust louvres on the windows as per the occupant's comfort level. And you could put solar cells on the louvres to help with the power requirements of the system since the 8051 can keep them normal (perpendicular) to the sun at all times during daylight.