AvalonEdit is a .NET WPF-based source code editor component developed for SharpDevelop 4.0 "Mirador".
AvalonEdit supports .NET 3.5 SP1 and .NET 4.0.
Source code can be downloaded from the SharpDevelop source code repository.
As SharpDevelop 4.0 is still in development the documentation for AvalonEdit is very sparse. All available materials are compiled in an article on SharpDevelop wiki.
Resources
Using AvalonEdit (WPF Text Editor)
Making AvalonEdit MVVM compatible