Over the last few months I've been working with Concordia's CART CGD group, which is a group of computer art students who want to make video games. We did a few things called "game jams", which is where a group of people meet together and try and churn out a game in a few hours. We didn't actually come up with anything that great this last semester, but then again there was only two game jams (if you're interested in participating, they have them every Saturday starting January 15th, send me an email and I can give you more details).
We did the game jams with Lua and a game framework called Löve2D, which helps simplify game development.
However, I found that I really liked Löve2D. A lot of things were very simple and straight-forward. Before that the nicest gaming library I worked with was pygame which is a wrapper around the C version of SDL and a few extra bells and whistles like collision detection and sprite handling. There's a port to Ruby called RubyGame, but I find it suffers from the same problem as pygame: it's just a wrapper around SDL with a few bells and whistles.
I decided to go and roll out my own Ruby gaming library called Rug. It's mostly written in C++ for speed, but the API is much more Ruby-ish than Rubygame or Pygame with a few hints from Löve2D. At the moment it doesn't have a lot of features; it supports some rudimentary collision detection and animations. It doesn't stick completely to Löve2D's API, there were a few things in there that I found kinda clunky when it came to animations that I fixed up.
You can see an example of Pong here with a screenshot:
Or a very simple platformer here, with a screenshot:
The library is still very new, and there are a few things that I'm not happy with that will change in the near future (in other words if you choose to fiddle with this, don't be surprised if things change - but then again if you've had any experience with Rails you will be used to this sort of thing ;) ). Documentation still has a ways to go, I started documenting the features religiously but in the end I found that I changed the way I wanted the library to work often enough that documenting can wait until things are a little bit solid. For example the physics module for collision detection has undergone quite a few API changes, and after having to update the documentation several times I decided that I will wait until I am happy with how the module works before telling everyone else how to use it.
So anyway, feel free to play around with it and tell me what you think. Installing it is pretty easy in Linux, however in Windows it can be a bit annoying with all the Ruby headers and the various SDL libraries. I managed to get it to compile in Windows, so shortly I'll put up a zip with all the various DLLs that you need to run Rug.
I have no idea how easy/hard it will be to set up on a Mac, if it works anything like Linux you can probably just use MacPorts to install SDL and the Ruby development headers.