RDT is an open source IDE for Ruby implemented as an eclipse plugin. So if you are familiar with eclipse platform, this is the best way to start Ruby development. RDT is written in Java along with some Ruby scripts to interface with Ruby code being written.
This tools is far from perfect. It features syntax highlighting, code completion and code formatting. It also provides a basic Test::Unit integration. RDT also comes with a simple regular expression tester.
RDT requires eclipse 3 or above version. You can download Ruby Development Tools here.