Heath Zenith SL-6180 wireless doorbell, in the process, figured out how to manually trigger the bells without using the remote. I then thought, why not have an arduino trigger the bells based on motion sense, floor pad sensor, or pushbutton. The door bell I'm using plays a very nice version of Westminster Chimes. Open it up, find the transistor labeled Q5 on the board and attach a wire to the base (center pin) to an arduino output. Connect a wire from battery negative to Arduino negative. When you want the chimes to ring, pulse the pin you have connected to Q5 - digitalWrite(pin, HIGH);
Other applications could be a audio notification when you get an email, a tweet, or completion of a task.