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; an Android port is being developed right now. The Ruby version works with MRI, MacRuby, and Rubinius, but not JRuby.
Gosu only provides basic building blocks:
- a window with a main loop and callbacks
- 2D graphics and text, accelerated by 3D hardware
- sound samples and music in various formats
- keyboard, mouse, and gamepad input
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Pros
Pro Lightweight
Gosu is not a game development framework, only a media library that happens to be suited to game development. (Kind of like SDL in the C world.) That means the interface is relatively small.
Pro Mature API, actively maintained and developed
Gosu has been under development since 2001. It is mature and has several toolkits built on top of it to provide additional functionality.

Pro Cross-platform, even mobile, using Ruby
Cons
Con Deploying Ruby apps is a mess
Games built with the Ruby to .exe "compiler" do nothing more than extract your source code and Ruby.exe to %TEMP%, then run it. The code is not really compiled at all. The process for wrapping games as Mac apps is a bit nicer, but you'll need a paid Apple Developer subscription to code sign the app, or users will see a warning/error when running your game.
The only way to really compile Ruby is to use RubyMotion, which does not work on Windows and requires a paid subscription on top of the Apple Developer one.
(This Con is not specific to Gosu. Deploying Ruby code has never been fun.)
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