iFactr is a cross-platform mobile framework created by ITR Mobility. With iFactr as a plug in to Visual Studio 2010, developers can create iPhone, Android and Windows Phone apps by writing C# and .NET code. Essentially iFactr compiles the code to a native app for a specified target platform. It is worth noting that the current version does require that the code be built and debugged on a Mac with iOS installed.
A bit history. According to Ben Horgen:
The C#/.NET cross-platform mobile framework started out natively compiling to iOS, Windows Mobile 6.5, and ASP .NET. The three targets provided the capability to deliver business applications to numerous mobile platforms from a single code base. No compiler directives, no interpretation, no fancy gui designer... just straight C#, .NET, and an abstract UI based on iBlocks, iMenus, iLists, and other UI constructs. From there the development team grew to nearly a dozen and the mobile framework started expanding to additional targets such as Android, Windows 7, Silverlight, and straight 4.01 HTML.
For more info on downloading and installing developer tools to create Windows Phone apps, read my previous post here.