One of the projects I've been involved with is the TFS Aggregator. It's a rules processor which allows you to do creative and crazy things whenever a work item changes in TFS or Visual Studio Tea Services.
People use it for calculating the Weighted Shorted Job First, automatically opening and closing items based on changes to their parent or children and more.
We recently released a rebuilt version of the Aggregator which removes a long list of bugs and offers a more powerful object model to build your rules in. At the moment we support VB.NET, C# and Powershell!
Instead of me trying to explain it all to you in this blog post, head over to RadioTFS to listen to the episode dedicated to the TFS Aggregator project.