The best open source alternative to Keyshot is LuxCoreRender, which is both free and Open Source. If that doesn't suit you, our users have ranked more than 25 alternatives to Keyshot and eight of them is open source so hopefully you can find a suitable replacement. Other interesting open source alternatives to Keyshot are appleseed (Free, Open Source), Sunflow Rendering System (Free, Open Source), Pixie Renderer (Free, Open Source) and Freestyle (Free, Open Source).
LuxCoreRender is a physically based and unbiased rendering engine. Based on state of the art algorithms, LuxCoreRender simulates the flow of light according to physical equations, thus producing realistic images of photographic quality.
appleseed is a modern, open source, physically-based rendering engine designed to produce photorealistic images, animations and visual effects. It provides individuals and small studios with an efficient, reliable suite of tools built on robust foundations and open technologies.
Pixie is an open source RenderMan renderer for generating photorealistic images. You can compile Pixie on Windows (using Visual Studio 2005), Linux and on OSX (using XCode or unix style configure script).
YafaRay is a free open-source montecarlo raytracing engine released under the LGPL 2.1 license. Raytracing is a rendering technique for generating realistic images by tracing the path of light through a 3D scene.
NOX Renderer is a rendering software based on unbiased methods. The renderer is available as a standalone Windows executable with a plug-in to link it to 3ds Max. A linking script is also available for Blender users.
MiniLight is a minimal global illumination renderer. It is primarily an exercise in simplicity. But that makes it a good base and benchmark (in some sense) for development and experimentation. And it just might be the neatest renderer around (on average, about 650 lines).