Scintilla is an embeddable text editor control that is based around the idea of TextFolding.
Author: Neil Hodgson Homepage: http://www.scintilla.org Family: ScintillaEditorFamily, OutlinerFamily License: Open source Platform: Windows, Linux, OSX
It is available on Win32, Linux, OSX, or any GTK+ platform.
Scintilla is written in C++, but has bindings for most major languages and applicable GUI toolkits. At this point, there are a number of text editors based on the Scintilla control, and it is the basis of the ScintillaEditorFamily here.
Screenshot: (for SciTE, an editor designed to demonstrate Scintilla)