Arnold is described as 'advanced Monte Carlo ray tracing renderer built for the demands of feature-length animation and visual effects'. There are more than 10 alternatives to Arnold for a variety of platforms, including Windows, Mac, Linux, Blender and LightWave. The best alternative is Adobe Dimension. It's not free, so if you're looking for a free alternative, you could try LuxCoreRender or POV-Ray. Other great apps like Arnold are appleseed (Free, Open Source), RenderMan (Free Personal), Mitsuba (Free) and Kerkythea (Free).
Adobe Dimension (formerly Project Felix and then Adobe Dimension CC) is the 3D design tool made for graphic designers. With extensive integration with Adobe Stock, you can easily composite 2D and 3D assets to build product shots, scene visualizations, and abstract art.
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.
The Persistence of Vision Raytracer is a high-quality, totally free tool for creating stunning three-dimensional graphics. It is available in official versions for Windows, Mac OS/Mac OS X and i86 Linux. The source code is available for those wanting to do their own ports.
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.
PhotoRealistic RenderMan, or PRMan for short, is a proprietary photorealistic RenderMan-compliant renderer. It primarily uses the REYES algorithm but is also fully capable of doing ray tracing and global illumination.
Physically based renderer. Mitsuba is an extensible rendering framework written in portable C++. It implements unbiased as well as biased techniques and contains heavy optimizations targeted towards current CPU architectures.