POV-Ray!

Sketchfab is the first web service to publish interactive 3D content online in real-time without plugins.

LuxRender is a physically based and unbiased rendering engine. Based on state of the art algorithms, LuxRender simulates the flow of light according to physical equations, thus producing realistic images of photographic quality.

Clara.io is a full-featured cloud-based 3D modeling, animation and rendering software tool that runs in your web browser.

Powerful, feature-rich and fast standalone raytracing renderer.

PhotoRealistic RenderMan, or PRMan for short, is a proprietary photorealistic RenderMan-compliant renderer.

visionaray - A C++ based, cross platform ray tracing library

A free Terrain Synthesizer and RendererPicogen is an outdoor ray tracing system, a free Terrain Synthesizer and Renderer targeting large-scale terrain with realistic day/night-illumination and detailed plant populations.

Structure Synth is a cross-platform application for generating 3D structures by specifying a design grammar. Even simple systems may generate surprising and complex structures. The design grammar approach was originally devised by Chris Coyne (for a 2D implementation see the popular Context Free Art).

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.

NOX Renderer is a rendering software based on unbiased methods.

Sunflow is an open source rendering system for photo-realistic image synthesis. It is written in Java and built around a flexible ray tracing core and an extensible object-oriented design. It was created as a framework for experimenting with global illumination algorithms and new surface shading models.Current Features:

Indigo Renderer is an unbiased, physically based and photorealistic renderer which simulates the physics of light to achieve near-perfect image realism. With an advanced physical camera model, a super-realistic materials system and the ability to simulate complex lighting situations through Metropolis Light Transport, Indigo Renderer is capable of producing the highest levels of realism demanded by architectural and product visualization.

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).