What's the first thing a thief does when he's casing a house? Ring the doorbell to see if anyone's at home, right? If someone answers, he can play it off like he's a door-to-door camera peddler or some kind of religious fanatic. If nobody answers, it's a good sign that the house is empty and ripe for the picking. The burglar proceeds to break in and steal everything that catches his fancy, including the ancient Chinese sword, the antique Italian restaurant menu, and the highly-valuable anime pencil-sketches. Meanwhile, the owner comes home to find all his shit missing, but there's nothing he can do because the perp is long gone.
Well, it's a different story when it comes to my house. Shortly after the thief rings my doorbell, I get an SMS message on my phone. I then rush home as fast as I can, where I find him struggling to lift the TV. I quietly grab my can of mace, sneak up behind him, yell "Surprise Motherfucker!", and get him good in the eyes (which are the testicles of the face). I am called a hero by my friends and neighbors.
This is how it works: My house has an oldschool mechanical doorbell. Normally, pressing the doorbell closes a circuit that activates an electromagnet which slams the chime against the tone bar. I bought a 12V reed relay from Radio Shack and hooked it up in parallel so that pressing the doorbell also activates the relay. This sends a signal to my computer (I'm glossing over this part because it's a bit complicated and involves a Phidget, not to be confused with a midget).
There's a program running on the computer that listens for the signal and makes a status update to Twitter user baic_house using Twitter's REST API. My main Twitter account follows baic_house and is set up to recieve an SMS message whenever baic_house makes an update. The whole process from pressing the doorbell to SMS message recieved on phone only takes a few seconds.