Gosu

Gosu is a 2D game development library for the Ruby and C++ programming languages, available for Mac OS X, Windows and Linux. The C++ version is also available for iPad, iPhone and iPod Touch.
Visual C++ 2008 (recommended) or 2010 required for the Windows C++ version, but the free Express versions are fine.
Both languages have access to:

• 2D graphics and text, accelerated by 3D hardware
• Sound samples and music in various formats
• Keyboard, mouse and gamepad input

Ruby/Gosu integrates well:

• gem install gosu
  complete with examples, precompiled for Windows & OS X
• gem install texplay
  pixel-based drawing on Gosu images, by banister
• gem install chingu
  a library that adds more game logic, by ippa
• gem install ruby-opengl
  the standard Ruby OpenGL gem works fine with Gosu to add 3D effects (see the included example, or this fine bundle of tutorials by tjbladez)
• gem install chipmunk
  a popular physics library with an integration example in the wiki
• gem install rmagick
  can be used as a more powerful but harder-to-distribute alternative to TexPlay, example included in Gosu

Ruby Game Development: Overview

If you are coming from a Ruby background and haven't yet decided on a library or want to know about alternatives to Gosu, or if you haven't yet decided to even use Ruby, check out Andrea O. K. Wright’s articles and conference talk.
Article (part 1) / Article (part 2) / Video of the talk (RubyConf '07)

http://www.libgosu.org/