99Sheep was written very quickly using the Kranzky Engine, which integrates Haaf's Game Engine (for graphics and sound), Box2D (for physics), Curl (for the online leaderboards), SQLite (for persistence) and XenonPad (for Xbox360 controller support). On top of all of that, it implements game state management, entity management and so forth.

We've been planning to extend the Kranzky Engine even further, by integrating Python, allowing all game logic to be moved into Python code. This would theoretically enable us to very quickly get working prototypes up-and-running, allowing us to test out game design concepts within a few hours, or a day or two.

I found it interesting to read today that 2DBoy, makers of World of Goo, have polished their rapid prototyping framework, and have made it available for download. Theirs is built on top of SDL, and stops shy of implementing entity management. It's release has definitely spurned me on to getting our own system up-and-running. Go team!