A magnetic reed switch is added to a door frame, magnet to the door. This is connected to a Pi's GPIO. When the door opens, a Growl push notification is sent to my iPhone.
Raspberry Pi UPS Monitor
I have an old UPS that doesn't communicate to Linux it's working status. So I hacked it's status lights to become inputs to a raspberry Pi's GPIO. Then scripted a bash program to monitor those lights. I get push notifications when the power goes
I added SiriProxy to a raspberry Pi, meaning I could ask Siri on my iPhone to open my garage door - or any other physical device connected to the GPIO. Here's a video: http://youtu.be/NUJ5z76Xv5o