The iPhone Garage Door Opener
Why? Cause it's cool to be able to press a button on my phone and have my garage door open. That, and, I have a horrible habit of leaving my garage door open all night for anyone to access, and now I can create a script to remotely close the door at a given time.
The stuff I bought consists of an NPort 16 port serial server (5610, if I remember) and an National Control Devices ProXR 4 Relay, 8 Input serial controller. To sense state, I included a garage door contact sensor with input into the ProXR.
On my MacPro, I have Tomcat running with a servlet which responds to the AJAX calls from my little iPhone-customized JSP page. The UI updates every 5 seconds, updating the door state and the current temperature in the garage (since a temperature sensor comes on the ProXR). So now I can use my iPhone to control my garage (which is almost useful)!