Ruby on Rails is a popular web application framework for a lot of modern websites. We've gathered 20+ tools to help you with your Ruby on Rails coding - feel free to add more in the comments.
ActiveScaffold.com - Ruby, Java, AJAX, all in one handy scaffold for better integration in your projects.
DZone Snippets - A repository of thousands of code snippets for different programs, tons for RoR.
Four Days On Rails - An attempt to teach you the basics of Rails in four days and get to where you can build basic apps in that time.
Gardens Point Ruby.NET Compiler - A Ruby on Rails compiler for the .NET system.
HoboCentral.net - An extension package for Rails that allows for templates, working with AJAX, and rapid deployment.
Instant Rails - Ruby, Rails, Apache and MySQL built in to one handy runtime for testing.
Learn To Program - An online tutorial for people who aren't that familiar with programming, let alone Ruby.
Locomotive - A Mac OS X application for RoR development.
Nuby On Rails - A guide to writing plugins for RoR.
Programming Ruby - An online book about learning the popular language.
Radrails.org - Home of Radrails, a downloadable, interactive development environment for RoR.
Ruby Cheatsheet - A detailed, printable, cheatsheet of the most common RoR commands.
Ruby Development Tools - A Sourceforge project, Ruby Development Tools is running on Eclipse.
Ruby Facets - A huge collection of core extension methods and standard library additions for Ruby.
Ruby In Steel - Ruby development that works with Visual Studio 2005. Does cost, but offers a free 30-day trial.
Ruby QuickRef - A reference site for everything related to Ruby on Rails.
RubyCentral.com - The central site for the Rubyconf and Railsconf.
RubyLearning.com - A tutorial site for learning Ruby that assumes you are at least familiar with programming in general.
RubyQuiz.com - Test your knowledge of Ruby with the Ruby Quiz site; you may even learn something.
RubyScript2EXE - A Ruby compiler which can strip down the Ruby library and embeds the interpreter in a EXE executable.
Try Ruby! - A hands-on, interactive tutorial of Ruby.
Yahoo Ruby On Rails Tools - Tools for using RoR with Yahoo and access to more APIs and applications. (via)