Tonight I stumbled across one of the coolest C# applications I've seen to this point located at http://worldwind.arc.nasa.gov/ (I'm probably a little late to the party on this one but better late than never I guess).  An extremely impressive use of managed DirectX, .NET, texturing, 3D imagery, plus a lot more.  The program is fairly large to install (170+ meg) but a lot of fun to play around with especially given that you can zoom into any area of the world and get different views of the landscape, landmarks, cities plus more.  It even lets you replay events such as earthquakes, storms, etc. and rotate around the world as it all happens.  They put together a nice FAQ section on why they chose .NET, C# and DirectX at http://worldwind.arc.nasa.gov/forums/index.php?showtopic=8&st=0&p=17&#entry17.