Heather Watkins by jeremy on Zooomr
Designer Li Jianye has designed a pair of doorbells for computer people (an Enter key) and for musical people (piano keys). I say, "why stop there?" How about a doorbell which sends you an SMS message when someone pushes it, so that if you are away from home, you would know about the event? It could have a little webcam to snap a picture of your visitor to send to you via MMS, and if you chose, there could be a little screen where you could message your visitor back.
I'm pretty sure that it could be implemented pretty easily with an Arduino Diecimila and a home computer.
Probably you would want an option to lock out the messaging functions if the doorbell gets pressed too often, to frustrate any annoying pranksters in your neighborhood.
Update: This was also posted on the Make: Blog. Also, I have an Arduino coming in the mail soon, so we'll see what doorbell-related tricks we might be able to have it do.