TfsAlert is a .Net 3.0 smart client which monitors for user selected TFS events and will display a balloon window in the Windows TaskBar Notification Area. It is built on top of WCF where you can subscribe to any TFS event you feel fit. It's initial focus is around Team Build management but will also include features around Work Item tracking.
You can get the code, but there's not a binary available yet. This project is the result of his desire to have something equivalent to the desktop notification tool used with CruiseControl.NET. Check it out -- and contribute to it!
[Update 11/2/2006] Clark has posted the first release!