A Ruby IDE written in Ruby
Ruber is a fully modular IDE for ruby written in ruby using korundum, the KDE ruby bindings.
It works on Linux (and should work on other Unix-like systems).
Except for the basic infrastructure, all of Ruber’s functionality is provided by plugins. This means that any user can easily augment Ruber’s features by writing his own plugin. He can also replace functionality provided by the plugins coming with Ruber in a way which integrates seamlessly with Ruber itself.
Ruber is written in ruby, and so, of course, are its plugins. This means that its users already know the language needed to extend it. A very different situation from, for example, Netbeans where you’d need to learn Java to write a plugin for programming in ruby (in other aspects, Netbeans is a good IDE, with very nice plugins for developing in ruby).
Ruber uses the wonderful KDE ruby bindings, which makes it expecially suitable for people using a KDE desktop (but can be enjoied also by users with a different desktop). In particular, Ruber makes use of the excellent Kate part for the editor window, meaning it has the extremely well-written ruby syntax highlighter and most of the tools Kate itself has.