Valkyrie is a 2D side-scrolling game engine written in C# using the Xamarin cross-platform framework. 1 common code base can be compiled to native assemblies for iOS, Android and Universal Windows (Desktop / XBox / Windows Tablet / etc..). That's a lot of market reach without the workload that usually comes from porting. Have a look at the repo on Github
Follow my progress as I design and write this engine on the Valkyrie Blog
If you would like to support open-source software consider donating to Valkyrie's Kickstarter campaign