RDT is an open source Ruby IDE for the Eclipse platform.
Features supported are syntax highlighting, on the fly syntax check, graphical
outline, Test::Unit view/runner, Ruby application launching, content assist,
source formatter, Ruby debugging, Type Hierarchy view, Ruby specific Search,
Refactoring, and much, much more.
A regular expression plugin is also available (Thanks to the EPIC project).

WWW: http://sourceforge.net/projects/rubyeclipse/