AvalonEdit
AvalonEdit is a WPF-based text editor component. It was written by Daniel Grunwald for the SharpDevelop IDE. Starting with version 5.0, AvalonEdit is released under the MIT license.
Download
Pre-built AvalonEdit binaries are available in the AvalonEdit NuGet package. The package contains binaries for .NET 3.5 and .NET 4.x.
The source code is available on GitHub.
Documentation
For introductory documentation, please take a look at the CodeProject article "Using AvalonEdit".
The source code download of AvalonEdit contains sources for a Sandcastle help file. The compiled documentation is available on this website.