I have an Ardunio lying around, currently they are not really doing anything (waiting for me to get off ass and order robotic goodness:) ).
As nice as it is to code them in the Ardunio IDE, it would be fun to code them in C#, so I could plug the sketches into other interesting things (currently I am intrested in using Touchless to control mobility robotics)
As it turns out there is a couple of things that allow me to do this:
Firmata.NET is a .NET implementation of the Firmata protocol.
Sharpdunio looks to be a slightly more verbose .NET implementation of the Firmata protocol.
PS: If you are in NZ and are looking to buy some Ardunio goodness for yourself, I would highly recommend Mindkits.
http://code.google.com/p/sharpduino/S