Related topic: Tools Supporting Watir
Eclipse
Eclipse is a popular, open-source IDE for many languages. It contains syntax highlighting, debugging and lots of other features.
- Introductory article by Neal Ford
- Installation and configuration of Eclipse
- RadRails - Eclipse Packaged with Ruby and Rails
Ruby Visual Debugging with NetBeans
IntelliJ IDEA
This commercial IDE for Java, also offers a Ruby plugin.
SciTE
"SCIntilla Text Editor" is an excellent, simple open-source editor that is installed with Ruby. Recommended for people who are not familiar with the above IDEs.
IRB, the "Interactive Ruby deBugger"
IRB allows you to instantly test/debug small pieces of code as you develop a Ruby/Watir script.