Writing samples and demos is tough. You want them to do more than say "Hello World" or draw a red square, but you want them to be simple enough that people can see the "new stuff" you're demoing (WPF, or Windows 7 taskbar integration) or multi-touch) in amongst the real business logic. It would be neat if they did something actually useful, because then you might keep it around on your machine and use it, but most things that are useful are too big to be demos.
Well here's a sample that lands in the sweet spot: Tasks.Show. You put in your tasks, things from your ToDo list, along with time estimates, and it keeps track of them and shows them to you. I like this view:
It uses touch to let you flick tasks into categories, and has taskbar integration to let you open a specific category, add a task, and so on. All the source code is available so you can see how it's done - it is a demo, after all. You can get more details and screen shots on the Windows Team Blog. Check it out!