Another option is Scintilla.NET (UPDATED https://github.com/jacobslusser/ScintillaNET).
It is a .NET 2.0 wrapper around the Scintilla Native Control. We have successfully used to display/edit HLSL, XML, Text, and other internal scripting languages. It is easy to write a language description file for your own language if needed.
I'm currently hosting the WinForms control in our WPF applications. The only difficulty was getting a WPF context menu to activate when clicking within the control; solved it by listening for a MouseRightButtonDown event on the WinFormsHost and then setting the ContextMenu visible (ContextMenu.IsOpen = true) :/
I have been in contact with Actipro Software re: SyntaxEditor and the WPF version is being actively developed and will available soon. For the time-being we will continue to use Scintilla.NET