Pixar released yesterday a non-commercial RenderMan version. They provide some information about it on their web site. Most of the people will use it from within Maya or Katana, but I want to provide some basic information about the stand alone renderer.
I installed it on OS X, but Linux or Windows versions should work in a similar way. First you should set an environment variable called $RMANTREE:
To render our first picture we will go directly into a subfolder and render one of the example RIB files coming with the free version:
RIB stands for the __R__enderMan __I__nterface __B__ytestream Protocol.
The resulting 512x512 image will be called oslball.tif as described in the RIB file:
It should look similar to the image below:
The patterns were generated via the OSL shading language and some examples (e.g. oak.osl) can be found in the shaders subfolder.