Here’s a quick round up of software development options that are either mentioned in the documentation or hinted at in the image.
This is the only currently supported development platform. The Windows IoT image supports only a limited subset of Win32 – so most existing applications will not run without modification. There’s a few extra APIs for Arduino functionality. The Galileo SDK has been made available as a Nuget package and has been open-sourced on GitHub
Silverlight (C# or Visual Basic)
The Windows IoT for Galileo image shipped with an embedded variant of Silverlight. It’s a .NET-style framework though, so not a repeat of the “Silverlight for Windows Embedded” that was present on Windows Embedded Compact, which was a native XAML presentation framework only very loosely based on Silverlight. It’s surprising to see Silverlight deployed here, considering the current tooling assuming that a Silverlight application must have a GUI, and that this framework seems superficially most similar to that currently found as Silverlight framework on Windows Phone – which is being phased out in favour of WinRT.