The Mono project is an open source effort sponsored by Novell to create a free implementation of the .NET Development Framework.
Mono includes a compiler for the C# language, a Common Language Runtime (CLR) for the Common Language Infrastructure (CLI) and a set of class libraries. The runtime can be embedded into your application. It implements both ADO.NET and ASP.NET.
It gets better. SharpDevelop is a free IDE (GPL even), and that’s where the strength of Microsoft’s various platforms has always resided.
develop (short for SharpDevelop) is a free IDE for C# and VB.NET projects on Microsoft’s .NET platform. It is open-source (GPL), and you can download both sourcecode and executables from this site.