Gosu: A Cross-Platform 2D Game Development Library for Ruby and C++
Gosu claims it's the "coolest gamedev library around." That's pretty hard to qualify, but with a focus on Ruby, I won't be one to argue. It's installable as a gem, with a simple gem install gosu, is MIT licensed (although the sound component is third party closed source), and works on OS X, Linux and Windows. It supports 2D graphics (accelerated with your 3D hardware), sound, and various forms of input (keyboard, mouse, gamepad, etc).
(Note: Sorry for the slow flow of posts on Ruby Inside this week.. I still have no broadband due to the house move, but it will be back on in the next few days.. and there's a great flow of Ruby news that has built up coming your way.)