GE-35 Christmas lights, hooked them into a .NET Micro Framework embedded controller, took another developer's work, developed a driver -- and voila! PC-controlled Christmas lights (video after the break).
In the demo, Andrej uses Microsoft Lync (Office Communications Server) to connect his instant messenger contacts (presumably Live Messenger) to the Christmas lights. The lights then show each contact's current status -- green, yellow and red -- or he can manually set them as 'out of the office', which is a rather pretty purple color. If the light is off, nobody is home.
While this is a cool hack, it pales in comparison to another computer-controlled Christmas lights experiment, which has been going since 2002: Alek's Controllable Christmas Lights. Alek lets you control over 21,000 lights, and inflate or deflate Elmo, Santa and Homer Simpson.