appleseed is a modern, open source, physically-based global illumination rendering engine primarily designed for animation and visual effects.
appleseed is actively developed by a small, international team of talented volunteers from the animation and VFX industry. Its core mission is to provide
... [More] individuals and small studios with a complete, reliable, fully open rendering package.
Over the years appleseed has been used on several projects including TV documentaries, ads, promotional videos and an animation short, "Fetch, a very short film": http://www.fetchaveryshortfilm.com/ [Less]
S-ray is a free photorealistic renderer based on monte carlo ray tracing.
It is capable of simulating all interactions of light with the environment, or L(D|S)*E paths in path notation. What this means is that it can simulate global illumination effects such as indirect diffuse illumination
... [More], caustics, and accurate reflection and refraction.
Here is a more or less complete feature list:
Indirect diffuse illumination and caustics. Reflection and refraction from perfect and imperfect (glossy) surfaces. Area lights casting shadows with penumbra regions (soft shadows). Motion blur. Adaptive supersampling. Hierarchical keyframe animation. Simple XML scene description format, with converters from popular 3D file formats.
A lot more features are planned for the near future, check out the ToDo list.
To start playing around with s-ray see: GettingStarted [Less]