Pixar is a whole universe, easily recognizable thanks to a unique style that can be found in all their works. And precisely, if you want a bit of this style, then today's news should interest you: it is now possible to download for free the rendering engine used by Pixar for all its films.
Named Renderman, this rendering engine is unique and the news has something to delight all 3D enthusiasts curious to try their hand at a new engine. But it will not go further: Pixar sets conditions on this provision.
Thus, Renderman is free but not free and it is therefore quite unnecessary to go and find its source code. But, more importantly: Pixar asks for a valid email address in order to download Renderman, and mostly asks for non-commercial use.
The trouble with the term “non-commercial” is that it can encompass very little like a lot, depending on who uses it. Here we are in the second case because, according to Pixar, even nonprofits raising money through registration fees or the like are among the commercial uses.
Anyway, if you want to personally have fun creating works through this renderer, without selling them afterwards, there's nothing stopping you. By having the right software.
Because if Renderman is available under Windows as well as OS X and Linux, it is not compatible with all 3D modeling software and the well-known Blender cannot therefore benefit from it. It will therefore be necessary to prefer proprietary and paid applications such as Maya, Katana or Cinema4D.