It's still pretty new, but Shattered Ruby looks interesting. It's a Ruby game development framework that runs on Mac OS X, Linux, and Windows, and uses the Ogre3D libraries (which support OpenGL and DirectX) for graphics. There's even a blog. It has some pretty neat event handling:

key :pressed => [:a, :b], :action => :do_something
def do_something
  puts "will be called when a or b is pressed"
end

I want to find some time to play with this..