DotGNU
DotGNU is a project for devising Web Services, software componentry and the like. It is compatible with Microsoft .NET and similar to the Mono project from Ximian.
The DotGNU developers say that while they realize that the Microsoft .NET platform is a powerful concept, its commercialization by a single company may be threatening the free software movement. There are nonetheless a lot of open source projects being made in C#.
DotGNU is able to compile and run .NET-compilable languages, like C, C#, Java, Perl, Python and PHP, among others.
It is a "language agnostic" platform in the sense that many languages can use one universal set of APIs.
As of this edit it appears that this project has not been completed, lacking aspects of the GUI.