Eclipse
Eclipse is a popular, open-source IDE for Ruby. It contains syntax hightlighting, debugging and lots of other features. This article by Neal Ford
describes Eclipse's support for Ruby.
Customizing .irbrc
IRB is a great tool to use to develop with Watir. These tips from Nick Sieger
describe how to make IRB even more useful.
Setting up Eclipse for Ruby
Steps to installation and configuration of Eclipse to efficiently execute Watir.
Visual Debugging with Ruby
NetBeans
Eclipse Packaged with Ruby and Rails
RadRails