jEdit is a popular Java based code editor. I have used it extensively and highly recommend it. Now you can also develop in Ruby using jEdit. And I am not just talking about syntax highlighting, much much more.

The jEdit Ruby Editor Plugin lets you manipulate Ruby and Rails code at the syntax level. Focused on programmer productivity, the plugin provides method completion and an integrated Ruby documentation viewer.

It allows variable refactoring, copy, cut, paste actions that operate on whole line when no text is selected, smarter autocompletion of brackets and quotes etc.

It is installable via the jEdit Plugin Manager.

It works with latest release 0.9 of JRuby & Java 5.0.

You can download it here.