So, Ruby is well know for its hugely popular gem Rails which greatly simplifies building websites. But what are some other programs built in Ruby off Rails? Here are a few very popular examples.
RubyMotion - Develop native apps for iOS (both iPhone and iPad) and OS X in Ruby. It is based on MacRuby which was developed by Apple. The app compiles into native objective-c just like iPhone apps made traditionally. RubyMotion (The Pragmatic Programmers) (Book)
Artoo - Gets Ruby up and running on Arduino, Leap Motion, Pebble, Rasberry Pi, Ardrone, Roomba and more. Built to let you program devices and the internet of things in Ruby. It also lets you connect multiple devices together through Artoo into an integrated robotic solution.
Rake - A task management tool that allows you to namespace tasks and run them with Ruby it is Ruby's alternative to make. A simple gem that you can install with 'gem install rake'. Built by Jim Weirich (who you need to know about), it enables you to run a command from the bash prompt and execute Ruby code inside your app or on the system at large. Wikipedia claims that "It is the most widely downloaded Ruby Gem, downloaded more than 40 million times and has been included with Apple OS X since at least version 10.7." Build Awesome Command-Line Applications in Ruby (Book)
Chef - A system provisioning tool that speeds up the initial configuration of servers as well as their maintenance. It hooks into cloud services like AWS and Rackspace. It works on the basis of "recipes" that define how a given task is to be done for example how packages are to be installed and what programs should be running on a server.
Capistrano - It started as mainly a rails deployment tool, but can be used to deploy any language webapps now. It sells itself as a remote server automation tool that can deploy to an arbitrary number of web servers and to automate common development tasks. Deploying Rails: Automate, Deploy, Scale, Maintain, and Sleep at Night (The Facets of Ruby) (Book)
Homebrew - A command line package manager for OSX that is a must have for any Ruby developer. Easily install databases like mysql and postgres or download python with 'brew install python' to try it out. It is powered by git and ruby so you can hack at it as you like. You can also create your own homebrew packages with 'brew create' and share them with the world.
Gosu - A 2d game creation gem that works on Mac, Windows and Linux.
Ocra - Creates Windows applications from Ruby code. Includes the Ruby code and interpreter in the application.