RenderMan is the core rendering technology behind not only Pixar's movies, but also a wide range of other Hollywood blockbusters. The rendering engine been used to generate visuals for everything from cartoony fare Toy Story to live-action titles like Edge of Tomorrow. Now, the professional-grade software is available completely free of charge for non-commercial use.
The latest release, version 19, promises the "highest quality global illumination and physically based visual effects, while also offering multiple types of light transport for unparalleled creative flexibility and artistic control." Pixar also claims RenderMan sets "new standards" for rendering speed and memory efficiency. It's important to note that this isn't a modeling or animation program, though. You'll need to bring your own assets and scenes created with third-party software.
The gratis version is fully functional and free of watermarks. The only string seems to be signing up for an account on the RenderMan forums. Although freeloading content creators are forbidden from taking in direct revenue from their projects, they can receive indirect revenue via advertising attached to their work. RenderMan needs to be credited for the rendering, but that's the only requirement.
If you want to try RenderMan yourself, the free release is available here for 64-bit versions of Windows, OS X, and Linux. I wonder if this might be a good benchmarking tool. Hmm.