#develop (SharpDevelop)
#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. In addition, you can find the latest information and changes on #develop, as well as get in touch with the team in the forum.

C# Studio
C# Studio is a simple IDE for a C#/Mono/GTK# developer. It has a standard "project/editor/output" interface.

Improve C# Plugin for Eclipse
Features: C# code editing with: Creation of new C# files (with a basic structure); Syntax highlighting; Content assistant for C# keywords; Auto-indentation. C# code compilation with: Compilation problems listed in the Tasks view; Compiler output information displayed in a console; Specification of the compiler arguments via a file property page (the arguments are persistent for each file). C# plugin parameters available via the plugin preference page: Path of Microsoft C# compiler; Automatic build on resource modification.

MonoDevelop
MonoDevelop is a project to port SharpDevelop to Gtk#. There are numerous goals that MonoDevelop hopes to achieve.

Sharp Builder Tools
Sharp Builder Tools is a free Expert for Borland C# Builder . Set of experts for Borland C# Builder. Contains a lot of tools integrated directly in the IDE like Ant / NAnt, AStyle, NUnit, todo viewer, some file management tools, some enhanced to the code editor, a file browser and a code browser.

Software Studio
Software Studio is an Integrated Development Environment (IDE) for C# and other languages with syntax higlighting, code folding, code completetion,refactoring support.