This cool system uses an Arduino board and home wiring to turn your local home telephone network into an intercom. When you take the phone off the hook it disconnects from the phone line and rings all of the phones with a different cadence. When you pick up the phone you get an open line so you can talk with loved ones and burglars who have broken in anywhere in the house.
The plans are amazingly complete and it could be a fun home project although it looks like it could take a while, especially if you electrocute yourself while wiring everything up.