Gosu is a nice simple 2D games engine for Ruby, which I used to work on this little game called pentix.rb. And I'd like to put in some good words for the folks who maintain the project.
What's particularly cool about Gosu is that it doesn't try to jump over its head and do everything by themselves. Gosu is a very small framework specifically made for 2D games, it will help you make things like windows, handle graphics, fonts, sounds and some basic animations. But it also easily integrates with Chipmunk to handle physics and with ImageMagic/OpenGL modules if you need some seriously looking graphics.
If you think about it, Gosu is a very cool thing, you leave all the rendering and heavy stuff to the native modules and use Ruby for what it's good for, for describing logic, units, interactions and so on. And Gosu does it right, it doesn't force you to inherit it's classes all the time, quite on contrary actually. Gosu itself has just a few basic classes to handle media things, like fonts and images, and for the actual units you can write plain Ruby classes, which certainly makes it much easier to implement, test and maintain.
And one more thing. Gosu is not just a Ruby toy. It also has all the C++ bindings, which kinda makes it a pretty interesting choice. You can use the power of Ruby to prototype and do all the R&D things. And later, if you actually make it to the investors, it will be relatively simple for you to port everything to C++ and make it running anywhere. Yup, iPhone is on the list :)
This is basically it. If you ever wanted to write some simple 2D games, but didn't want to deal with C++ check out Gosu, it's pretty awesome!
UPD: Check out also this simple ruby tutorial they have