Their build light is updated based on the continuous integration status for all active projects (currently 12 project).
The continuous integration server is triggered anytime code is pushed to our master git server.
Green = All projects built successfully and all unit tests are passing.
Red = Projects could not compile/did not pass unit tests.
Yellow = Projects are currently being built.
The second-runner up (and the winner of an iPad mini) is Toby Weston - @jamanifin
That's a real (ill gotten) traffic light with accompanying finer grained indicators on the monitors either side.