Game Blocks is a free, open source library for making games. A purely visual programming toolkit, it was built with BYOB, which is built on top of Scratch.
Game Blocks was developed as a teaching aid by game developer Sheldon Pacotti for a course at the University of Texas entitled "RTF 344M: Writing and Narrative Design for Video Games." The library allows novice game designers to experiment with storytelling, animation, physics, interaction and standard game types like adventure games, platformers, and arcade shooters. Projects can be compiled into executables for Mac, PC, and Linux. The library itself, running within BYOB, is cross-platform as well.