Geo Transformer (GT)

GT is an SDK with a visual component which implements most of it's core functions.

The purpose of this tool is to transform sets of points in either projectional or geodetic format from one coordinate system to an other. It's database is filled with almost every possible coordinate system, but you still can add more or even create your own (SDK only). 

You can use it as it is as a stand alone application, embed it in your own app, or just use the framework's functions to achieve the best result for your needs.

It's written entirely in .NET (Visual Basic, still can be used in C# or any other .NET language), source code is also available on demand.