We've seen a lot of DIY home automation projects over the years, but Instructables user electronichamsters shows off one of the most complete systems we've seen yet. You'll be able to use it to monitor just about everything in your house.
Instead of concentrating on the simple things such as lights or automated blinds, elecronichamsters goes all in. With his system you can monitor for water leaks, see if the garage door is open, check for new mail, watch for movement, sense for gas, and even see how the dog is doing. The system uses both an Arduino and a Raspberry Pi alongside various sensors to monitor the house. You can set up the system to send you alerts when something happens or just monitor everything from your phone.
Uber Home Automation w/ Arduino & Pi [Instructables]