X3D is the open-standard format for 3D graphics scenes on the Web.
rayshade and POV for Mathematica Export + view
An X32/M32 scene to Wing snapshot converter
Converts between many 3D data formats
Rich HTML UI engine for UE4
An application that create a scene in HGE file format.
1st 3D game engine built in C# for .NET, Visual3D Engine's predecessor