I am searching for a new TDD example

I am looking for a new example to demonstrate Test Driven Development. In the TDD book that Alexei and I wrote we demonstrated TDD using a Stack. For a copy of the chapter see http://workspaces.gotdotnet.com/tdd. We felt that this was a good example to demonstrate how to do TDD. Since then we have received some feedback from people who bristle at the Stack example because most people don't write Stacks and want an example that is closer to what they work on day-to-day. 

However the difficulty comes in when trying to do something real-world is that you end up explaining the problem more than the process. This is why I felt the Stack was a good example because very little problem description is needed and most of the time is spent focusing on the issues related to TDD. The other issue and one that I feel is more compelling for changing the example is that I would like to have an example that demonstrates collaboration between objects.

So your task, if you wish to accept it, is to come up with an example that does not take a great deal to explain, demonstrates collaboration between objects and could be done concisely in 3-4 hour demonstration.

Thanks for your help in advance.