.net平台下开源（免费）三维（地形，游戏）GIS平台
open source (free) 3d (terrain，game) gis platform based on .net (C#)
Axiom：开源3D游戏引擎，是著名的开源引擎OGRE的.NET版本。源代码完全用c#构造，主要为图形部分。
RealmForge：开源3d游戏引擎，图形部分采用Axiom、并组合并重写其他一些网络、人工智能等模块，完全由C#构造。
还有google earth，
Axiom
The Axiom 3D Engine is an open-source, cross-platform 3D graphics rendering engine for .NET and Mono. The engine is a high-performance C# port of the powerful OGRE engine. Its flexible component-oriented architecture allows easy extension and provides full support for both DirectX and OpenGL. The engine is also cross platform supporting both Windows and Linux operating systems.
RealmForge：
The .NET 3D game engine predecessor to Visual3D.NET (www.visual3d.net), which is a framework and toolset for the visual design and development of 3D games, simulations, and interactive environments using C#, .NET 2.0, and XNA.
irrlicht
The Irrlicht Engine is an open source high performance realtime 3D engine written and usable in C++ and also available for .NET languages. It is completely cross-platform, using D3D, OpenGL and its own software renderer, and has all of the state-of-the-art features which can be found in commercial 3d engines.
We've got a huge active community, and there are lots of projects in development that use the engine. You can find enhancements for Irrlicht all over the web, like alternative terrain renderers, portal renderers, exporters, world layers, tutorials, editors, language bindings for java, perl, ruby, basic, python, lua, and so on. And best of all: It's completely free.
非.net平台
ORGE：评价非常高开源3D游戏引擎。使用c++构建，主要为图形部分。
TerraVision
http://www.ai.sri.com/digitalearth/index.shtml
http://www.ai.sri.com/tvgeo/download_full.shtml
https://sourceforge.net/projects/terravision
TerraVisionTM is an Open Source distributed, interactive terrain visualization system developed by SRI International. It allows users to navigate, in real time, through a 3-D graphical representation of a real landscape created from elevation data and aerial images of that landscape. There are many terrain visualization tools on the market, but here are some of the features that make TerraVisionTM unique:
New: Full Open Source hosted on SourceForge.
TerraVisionTM can browse huge datasets, in the order of terabytes.
All data can be distributed over multiple servers across the Web.
3-D VRML and GeoVRML models can be overlaid, e.g. buildings, wind vectors, etc.
TerraVision can access OGC Web Map Servers (WMS) [More]
Datasets of different size, resolution, source, and coordinate system are supported.
New: TerraVision Server on Internet2.
Available for free for Windows, Linux, and SGI platforms.
New: Version 6.0 released