NStudio is a free (open source) editor that was written in C# on the .NET 2.0 Framework.
The idea of NStudio is to support all the features of one perfect text editor.
NStudio has 5 modules for editing text (Scintilla, Rich Text Editor,ICSharpCode editor, Standart windows editor and WYSIWYG HTML Editor),web browser, plugins and many tools.
Go to http://nstudio.sourceforge.net/
See also these related projects.