Accelerometer, am I upside down?
Travis Feirtag shows how to use an accelerometer and .Net 2.0 in 2D and 3D. To do the rendering, Travis uses GDI+ for the 2D example and Direct X for the 3D example. For an accelerometer, Travis uses the Analog Devices XL202 Evaluation Board with an RS232 interface.
Now what can you do with an accelerometer? You can build a WiiMote, determine if you're in motion or upside down.