What do you do when you have an arduino, a camera, an ethernet shield and a doorbell? You make your own intelligent security system.
The system is actually pretty simple. When the doorbell rings an Arduino sends a request to a notification service called PushingBox which then grabs a picture from web camera located outside. Then PushingBox sends a notification to an iPhone (it looks like this can be modded slightly with PushingBox to include Android and Windows Phone as well) and an email with a picture attached.
[original story: Arduino Blog]