Platinum Arts Sandbox is the game design tool based on the Cube 2 engine that allows users to quickly and easily create and edit their own worlds in game, even cooperatively. It is free, open source, and easy to use for Kids and Adults!

The project is to remain open source and free so that developers and kids can help add new content and features to help make Platinum Arts Sandbox the most fun and easy world creation tool possible.  Platinum Arts Sandbox will be easy enough for kids to use but also powerful enough for full game projects.

Main Feature List:
– The ability to map edit in game in real time and cooperatively with other people.
– The ability to immediately play with and interact with map editing changes
– Many Models to drop into the map including trees, food, tools, structures, etc.
– Several custom maps that will feature slides, jumping levels, mazes, amusement park rides, etc
– The pursuit of making it as easy as possible to add map features and content.  Also the addition of new entities to add even more features and capabilities.

Download Version 2.2.4: Windows Linux or Macintosh

Game tutorial

Apparently (if i understand correctly) one of the author of Platinum Arts, really enjoy Granado Espada (a free mmo) and its soundtrack.