Eric is a full featured Python and Ruby editor and IDE, written in Python. It is based on the cross platform Qt GUI toolkit, integrating the highly flexible Scintilla editor control. It is designed to be usable as an everyday quick and dirty editor as well as being usable as a professional project management tool, integrating many advanced features that Python offers the professional coder. Eric includes a plug-in system, which allows easy extension of the IDE functionality with plug-ins downloadable from the net. Current stable versions are Eric4 based on Qt4 and Python 2 and Eric5 based on Python 3 and Qt4.
Arcadia is a Light Integrated Development Environment (IDE) for the Ruby language written in Ruby using the classic Tcl/Tk GUI toolkit. Some features include an editor with source browsing, syntax highlighting, and code completion, debug support, the ability to work on any platform where Ruby and Tcl-Tk work, a highly extensibility architecture, and support for RAD GUI building.
Diakonos is a customizable, usable console-based text editor. It features arbitrary language scripting, an interactive help system, bookmarking, regular expression searching, parsed ("smart") indentation, macro recording and playback, a multi-element clipboard, multi-level undo, a customizable status line, completely customizable keyboard mapping, and customizable syntax highlighting.
AEditor is a programmer's editor for Windows, Unix, and Mac OS X that relies on the fox toolkit. It can do syntax coloring of Ruby and C++ files. It's easy to change the color theme. Most things are customizable. It is written in Ruby and is carefully unit-tested (it has 421 tests). It is written in only ~5500 lines of code, so it should be easy to grasp and extend.