Inspired by TheOm3ga's C++ port of Robert Penner's tweening code, I decided to port Penner's code to Ruby.  I ported all of his easing code, and even his demo.  You can find more about it at the link below (it has screenshots too), and install it for yourself with "gem install tween".  The demo is in the gem's example directory.

http://ruby.about.com/od/gosugme/ss/Smooth-Motion-Tweening-In-Gosu-With-Ruby.htm

And it's on github here.

http://github.com/mmorin/tween