Greg Borenstein used an Arduino with an Ethernet Shield and a few relays to make this stoplight for the folks at GitHub. He writes:
A couple of months back, the guys at GitHub bought a stoplight. They ordered it on Ebay from a seller in the UK. Their plan was to hook it up to their build system.
The idea was that when they check code into their continuous integration system (see defunkt/CI Joe) the stoplight would turn yellow while the code was building, red if the build failed because of failing tests, and green if the build succeeded. It would give them a large unmissable indicator in their office for the status of their current code.
You can read his great write-up of the whole project here.