Pixar has decided to make Renderman free for non-commercial use. Renderman is the professional rendering solution par excellence, as used in Disney productions.
Renderman needs to be used with a 3D modeler. The two modelers it's 100% compatible with, is Katana, which is available only on linux, certified for use with Red Hat products, and of course Autodesk Maya, which is also best used in linux, for performance reasons. Compatibility with Blender is in the pipeline for a free solution in the very near future.
This is quite a big deal, because it means that creators without a budget, can use high end tools for reasonable budgets, even for professional purposes, because the commercial license only costs about 500 bucks, which is very reasonable in my opinion. Katana and Maya are of course also commercial solutions, but they are proven to work very well. In the near future though, Renderman will also be compatible with Blender, the free and open source 3D modeling application, that comes by default in quite a number of linux distros, and is present and maintained in most linux repos.
There is also limited but very usable compatibility with Side Effects Houdini, which is also developed for Windows besides for OSX and Linux. There might be other 3D modelers that develop compatibility with Renderman now that Renderman is easily available.