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++ 2005 or 2008 needed 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

• A comprehensive translation effort into French. Also, translations of the Ruby tutorial into Traditional Chinese and Spanish as well Polish and more French material—see DocsOverview in the Wiki
• Installation via gem install gosu, complete with examples
• An example for using ruby-opengl to add rich 3D effects to 2D Gosu games
• A tutorial on how to use the Chipmunk physics library with Gosu: RubyChipmunkIntegration, written by Dirk Johnson
• RMagick integration, shown in an example game—or you can use TexPlay, a light-weight replacement tuned for games

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. Wrights articles and her talk, which give a fair overview.

Article (part 1) / Article (part 2) / Video of the talk (RubyConf '07)

Licensing

Gosu and all documentation © 2001-2009 Julian Raschke, Jan Lücker and contributors.

Gosu is released under the MIT license.

Note: Even though Gosu's own source code and documentation are released under the MIT license, it still uses the closed source FMOD library for sound output, which is free of charge only for use in freeware.
On Linux, SDL_mixer will be used as the default. You use 100% Free code in this case!