Open Source Text and Code Editor
Microsoft Visual Studio Editor API definitions
FlashDevelop is a free and open source code editor.
Open Source Editor and API for creating games in Flax Engine
Open Source Multiplatform Game Engine, C# OpenGL Version (custom wrapper) including the Editor
The WPF-based text editor component used in SharpDevelop
FastSharp is a text editor which lets you quickly compile and run C#, F# and Visual Basic code without opening up Visual Studio.