AvalonEdit is the name of the new WPF-based text editor in SharpDevelop 4.x "Mirador"
Source code: You can download AvalonEdit as part of SharpDevelop 4.x from our git repository or from the build server. These builds target .NET 4.0, but the AvalonEdit source code can can still be compiled for .NET 3.5 SP1 (change the target framework and remove the DOTNET4 conditional compilation symbol). You will need to run 'debugbuild.bat' for compiling SharpDevelop so that GlobalAssemblyInfo.cs gets generated.
The source code download of AvalonEdit contains sources for a help file (created with Sandcastle, if you don't like reading the XML sources). The compiled help file is available as download on the CodeProject article.