MiniLight is a minimal global illumination renderer. It is primarily an exercise in simplicity. But that makes it a good base and benchmark (in some sense) for development and experimentation. And it just might be the neatest renderer around (on average, about 650 lines). There are translations into several programming languages.
It is a command-line application. It reads a simple text model file and writes a PPM image file. Open-source C, C++, Scheme, Scala, OCaml, Lua, Python, Ruby, and Adobe Flex for Mac, Linux, and Windows.