Use an Arduino and an optional IP camera to build a doorbell that sends you a Push notification on your phone and an email with a picture of the person at the door.
This project will help you to hack your doorbell to send Push notification and an email with an attached picture when somebody’s at the door. It use the free PushingBox service to delegate all the programming part and make things easy.
UPDATE (July 2013) : For those who don’t know how to program an Arduino, there is a new Internet Of Things wireless device called Notifon, preprogrammed and ready to use. More information at http://www.notifon.net.