I've done several projects on Windows Mobile using the .NET Compact Framework, Microsoft's framework for writing mobile applications. At Locomatix, I've been developing a mobile client for our My Aura application, which includes location sensing and instant notifications. At the Whereabouts Project, I maintained a student-developed tour guide application, which we used to give tours of our computer science buildint to visitors.
.NET is a very powerful framework, and the Compact Framework is a nice way to develop for a mobile device. I've really enjoyed the challenges of working on a resource-constrained platform, dealing with sporadic network connectivity, and solving user interface problems on such a small device.