MonoDroid is a .NET development system targeting the Android mobile platform based on the Mono project's MonoTouch tool. Mono is all about porting .NET to other environments - Linux, OS X and so on and even Windows. MonoTouch is a development environment for the iPhone which uses the .NET Framework and languages. At the moment it represents the only non-Apple way of creating apps for the iPhone and iOS 4 in general. So far Apple don't seem to have found a way to object to its use or perhaps they just don't see it as a major development route. You still need a Mac and the iPhone SDK to use MonoTouch.
MonoDroid is a similar .NET based mobile development system but targeting the comparatively open Android environment. Currently Windows Phone 7 is the only mobile platform other than the iPhone/MonoTouch system that .NET programmers can target.
Read more: I Programmer