On my free time I've been working on a tool to prototype gameplay ideas - it's basically an easy-to-use editor environment with wide format support for models, textures and audio files that's scriptable using AngelScript. I wanted something that's easy to use rapidly and flexibly enough so I don't have to write small throwaway apps with lots of boilerplate code to test a gameplay idea that I have. The tool is OS X only (and I'm not planning on porting it to Windows, sorry) and I posted a short video of me using it (along with a few scripts I've written and a DualShock3 controller) to create a simple interactive environment:

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I'd appreciate any feedback you have about it, and if you're interested in using it to work on your own game idea or as an editor for your game (I plan to create an exporter API and would love to have some actual use cases for it) why don't you get in touch with me? I'd be willing to implement the features you need and provide you with weekly builds.