Canvas Painting
I have begun prototyping my game ideas in canvas (the HTML5 element). It is faster to build a prototype in JavaScript than C++, anyway. I am sticking to 2d prototyping for now, but when I get my head around WebGL, I should also be able to churn out 3d prototypes.
Canvas is readily supported and automatically cross-platform, as well.
All my current and future JS prototypes are available here: js.zachanima.net