My code for a simple home automation setup based on cheap Arduino
Pro Mini clones, relays and nRF24L01+ modules.  It will also work
as a simple wireless sensor network and perhaps collect statistics
about the use of each light.

http://wiki.makespacemadrid.org/index.php?title=ArduinoMicroWSN

There is little actual automation here, it replicates the original
setup with working wall switches.  Each wall switch is connected to
digital ground and an Arduino GPIO through a long cable.  Lights are
connected to the mains through a relay driven by another Arduino
GPIO.