Most of the 3D apps have realtime shaders for previewing in the viewport. Then you have Cryengine, UDK, Unity and a bunch of other mod tools that are available.
I'd say look at the Unity asset store. There are assets and things up there for free and for sale. As long as there isn't copyright infringement going on it should be fine.
thanks for the links and example, CrazyButcher. :) another question for ya, Ninjas...did u guys use Unity for the dev, or program it with your own tools?