RADRails
RADRails was one of the first serious Ruby-specific IDEs. Despite the name, RADRails is not only for Rails applications. It, in fact, is generally useful for developing Ruby applications (see Figure 1). Based upon the popular Eclipse IDE, RADRails is cross-platform (Windows, Linux, and OS X) and open source. Although other IDEs have now become popular, RADRails is still a good Ruby-specific choice.

jEdit
Like RADRails, jEdit is an open source, cross-platform IDE. Unlike RADRails, it isn't Ruby-specific at all. It is a general programmer's text editor. What earns jEdit a spot on this list is its "Ruby Editor Plugin," a plugin that adds a number of Ruby- (and Rails-) specific abilities to the editor, including syntax and error highlighting, integrated documentation, and auto-indentation (see Figure 2).

Find more information about the Ruby Editor Plugin at rubyjedit.org.

Ruby In Steel
Ruby In Steel is a professional-grade Ruby IDE for Microsoft Visual Studio (MSVS) 2005. It features not only code completion, but also full Microsoft-style IntelliSense features on Ruby code (see Figure 3). While it's not cheap ($199US), a free limited-feature edition and a free thirty-day trial make Ruby In Steel appealing to new Ruby developers who particularly appreciate the MSVS IDE.

TextMate
TextMate is an editor available only on Mac OS X. Its use by most of the core team of Rails developers has led to its strong adoption among OS X-based Ruby developers. Like jEdit, TextMate is a general programmer's text editor with a significant number of available Ruby-specific extensions. Depending on the current exchange rate, TextMate costs approximately $50US. TextMate's developer, Allan Odgaard, has been helping another developer produce a Windows clone called E (a.k.a. E-TextEditor).