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.
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.