nngn is a free and open source 2D/3D graphics/game/physics engine. Read more about it in the blog section.
Try it without installing or compiling anything here (or here for a guided demonstration).
There might be issues that are not present in the native build. The console can be informative if things go wrong. The page will also download approximately 10MB of data.
Watch an earlier version of the demo here or on Youtube.