Quest 3D is a pretty interesting tool for building 3D virtual worlds. Available in several versions with a range of features and costs. I have a few friends who use the tool for building walk-thrus of architecture projects. Website includes screenshots, movies and .exe demos of sample work. The worlds produced are visually rich with the lighting, texturing and shape features that one expects in modern tools of this category. I'm not sure how strong the physics model is though. Literature says "physics simulation" but I didn't find too many details about that. From what I found, this focuses on lighting effects and positional sound. Doesn't appear to support mass, friction, particles, etc.If you download the samples, be forewarned that most of the MPG captures are kind of grainy - you get a better idea of the visual quality from the screen shots and the .exe demos.