Kidding with my friend Sephiroth, i've tried ScintillaNET, .Net version of famous Scintilla, a library for advanced code-editing, code folding and highlighting.
Nice tool, easy to use!
ScintillaNET Home

If you want to try another similar tool but very complex, you could download the source code of SharpDevelop, an IDE entirely written in C#, and dissect it to find the control.
SharpDevelop Home