Currently, I have found Scintilla. It has a Cocoa port, but I am not sure how stable it is. And I think it is bloated. Any simple, cross-platform edit control with syntax highlighting (and possibly 'code folding')?
Note: No need for a GUI. Just the 'guts' are fine.
Update: I was actually mistaken somewhat. I was confusing 'Rich Edit' control with 'Code Editing' control. Scintilla is a 'Code Editing' control, and that is what I am interested in. A control that has the best of both worlds would be icing on the cake.