Open Source Renderers:
Mitsuba is a research-oriented rendering system in the style of PBRT, from which it derives much inspiration. It is written in portable C++, implements unbiased as well as biased techniques, and contains heavy optimizations targeted towards current CPU architectures.
In comparison to other open source renderers, Mitsuba places a strong emphasis on experimental rendering techniques, such as path-based formulations of Metropolis Light Transport and volumetric modeling approaches.
LuxRender is a rendering system for physically correct, unbiased image synthesis.
It supports both unbiased (MLT/(bidir)Path Tracing) and biased techniques (directlighting, photonmapping), physically correct light sources, advanced procedural textures, spectral light calculus, motion blur, lightgroup blending and tight integration with blender.
Yafaray is a ray tracer, offering Global Illumination and Photon Mapping, for high quality photorealistic rendering.
Focusing on stability and production usage, features include constructive solid geometry, depth-of-field, extensible shading engine (DSOs), instancing, level-of-detail, motion blur, NURBS, procedural plugins, programmable shading, subdivision surfaces, subpixel displacements and more.
Blender integration is handled by the MOSAIC plugin, which allows rendering to all popular RenderMan renderers.
POV-Ray, is the oldest free and open-source 3D renderer. It generates images from a text-based scene description language. It supports mathematically defined primitives as well as their polygonal counterparts; A Turing-complete scene description language (SDL) that supports macros and loops; Subsurface Scattering (sss) and translucency; Atmospheric effects such as fog and media (smoke, clouds); photon mapping; Pause and restart a render after shutdown; Real Time Rendering mode, etc.