Wikipedia
SharpDevelop (also styled as #develop) is a free and open source integrated development environment (IDE) for the .NET Framework, Mono, Gtk# and Glade# platforms. It supports development in C#, Visual Basic .NET, Boo, F#, IronPython and IronRuby programming languages.
It was designed as a free and lightweight alternative to Microsoft Visual Studio, and contains an equivalent feature for almost every essential Visual Studio Express feature, including features for project management, code editing, application compiling and debugging. To allow for easy project migration, SharpDevelop works natively with Visual Studio project and code files. It is able to compile applications for .NET Framework version 2.0, 3.0, 3.5, 4.0 and the .NET Compact Framework 2.0 and 3.5.
Although not as widespread as the Visual Studio line of products, SharpDevelop is fairly popular and has been downloaded at least 8 million times worldwide, and has been documented in the book Dissecting a C# Application: Inside SharpDevelop (2003) published by its creators.
SharpDevelop is written entirely in C# and consists of about 20 components that integrate to form the application. The source code editor component is known as AvalonEdit and can be used by other applications. Early in its development the project was split for Mono and Gtk# development into the MonoDevelop project.