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.
Evergreen is a cross-platform development environment that tries to be lightweight and language-agnostic yet functional. It started as a project to reimplement Rob Pike's Acme editor for Plan 9 in Java, but has since evolved in directions that help it deal with large codebases and multiple projects/branches at once. Remaining similarities include the tiled windows and the Unix-like reliance on external programs rather than reinventing every wheel. The major philosophical differences include strong support for keyboard-based editing, language-specific functionality, and native platform UI conventions. There are also two new guiding principles: accepting regular expressions and output diffs.
GNU TeXmacs is a free wysiwyw (what you see is what you want) editing platform with special features for scientists. The software aims to provide a unified and user friendly framework for editing structured documents with different types of content: text, mathematics, graphics, interactive content. TeXmacs can also be used as an interface to many external systems for computer algebra, numerical analysis, and statistics. New presentation styles can be written by the user and new features can be added to the editor using Scheme.