Continuous integration is the process that continuously build, analyze and test your sources. In many cases the process is triggered when changes are notified in the version control system, like VSS, CVS, etc. Martin Fowler has a good article about continuous integration.
In the .NET world, the most famous tool is CruiseControl.NET in combination with NAnt & NUnit. Getting an e-mail or popup from CruiseControl.NET is nice when a build is broken, but notifying the build status through traffic lights is much cooler.
Michael Swanson integrated CC.NET with the Ambient Orb. I think that the Ambient Orb is not an option for Europe, but you can integrate by using the X10 home automation technology. A good article about integrating X10 with .NET can be found on Coding4Fun and is called Controlling Lights with .NET.
Here are some (other) implementations:
I am going for a walk this evening, and I think that tomorrow a traffic light will be missing 
