With the merge of RadRails into Aptana, the Aptana+RadRails bundled IDE now supports Ruby syntax highlighting, debugging, code completion, and support for Rails-specific tasks like defining WEBrick and Mongrel servers. Although the project is still in beta, it is already very usable for daily use.
Alternatives: EasyEclipse for Ruby on Rails, which is very easy to install (bundles the Java runtime) has RadRails built-in and you can enhance it using the Aptana plug-in. Sun's NetBeans 6 IDE bundles Ruby support (currently pre-release, but usable.) SciTE has built-in Ruby support and loads very quickly.