I've been using the SharpCode.TextEditor for a few cases, and it works quite well - including syntax highlighting and all.
Check out this Using ICSharpCode.TextEditor article on CodeProject for an intro.
To download it, go to the SharpDevelop web site and download the latest sources. One of the projects included is the Text editor, which you can easily isolate into its own assembly or sub project - you get all the source code, after all!
