Here are some render comparisons for the curious: http://appleseedhq.net/stuff/renderers-comparison/index.html
- Mitsuba is a mainly-academic renderer by Wenzel Jakob (I say mainly because I haven't seen its widespread use in non-academic setting).
The two important open source renderers that are missing in comparison:
- Cycles (comes with Blender; produces great results although it is said that it only recently has started doing a "PBR workflow")
- Luxrender (very popular with artists on deviantart; has very good OpenCL support)