The Axiom Engine Project is a fully object oriented game engine development effort using C# and the Microsoft.Net or Mono platform to create an easy to use, flexible, extendable, and powerful engine that allows for rapid development of games for various genres....

SharpMap is an easy-to-use mapping library for use in web and desktop applications. It provides access to many types of GIS data, enables spatial querying of that data, and renders beautiful maps.

ASP.NET is a free web framework for building great web sites and applications.

This site is the home of the ASP.NET MVC, Web API, and Web Pages source code. If you want to use the released versions of these products to develop your applications, visit http://www.asp.net to find official installers, documentation, tutorials, samples, and videos....

The Open Toolkit (OpenTK) is a free, fast, cross-platform C# wrapper for OpenGL, OpenGL ES, OpenAL and OpenCL.

The bindings are strongly-typed and contain inline documentation for improved security and coding speed. Additional utilities integrate the bindings with .Net, making OpenTK especially suited to Rapid Application Development....

Umbraco is a .NET-based CMS+ that let's all people involved in creating and managing web applications easily collaborate and have a clear view of can be and needs to be done.

For developers:
Full .net 4.0 integration, webservices, extendable XSLT, the list goes on. ...

A mathematical open source library written in C#/.Net, aiming to provide a self contained clean framework for symbolic algebraic and numerical / scientific computations.

The OpenMetaverse project is a collection of building blocks to construct the next generation of virtual world platforms on. Born out of the libsecondlife project, it contains a reimplementation of the Linden Lab Second Life protocol, and will host future metaverse implementations and enhancements as well as examples and documentation....

Quartz.NET is a full-featured, open source job scheduling system targeting the Microsoft.NET platform. Quartz.NET can be used from smallest apps to large scale enterprise systems.

The Json.NET library makes working with JavaScript and JSON formatted data in .NET simple.

Xapian is an Open Source Search Engine Library, released under the GPL. It's written in C++, with bindings to allow use from Perl, Python, PHP, Java, Tcl, C#, Ruby, Lua and Erlang (so far!)

Xapian is a highly adaptable toolkit which allows developers to easily add advanced indexing and search facilities to their own applications. It supports the Probabilistic Information Retrieval model and also supports a rich set of boolean query operators....