If you want to experiment with Physically Based Rendering, PBRT is the tool to use. Unfortunately, the latest builds (>1.03) have some incomplete instructions on how to build PBRT on your own machine. I’ve built it on Ubuntu 9.10 and 10.04 using the following instructions:

• PBRT needs the OpenEXR libraries, but unlike it mentions in its makefile, just installing openexr and libopenexr-dev is not enough. The full apt-get install line should read:
  sudo apt-get install openexr openexr-viewers libopenexr-dev libilmbase-dev.
• The makefile looks for EXR libraries in /usr/local/*, while ubuntu installs them in /usr/lib. The following lines in the makefile should be changed:
  EXRINCLUDE = -I/usr/local/include/OpenEXR to EXRINCLUDE = -I/usr/include/OpenEXR
  EXRLIBDIR = -L/usr/local/lib to EXRLIBDIR = -L/usr/lib
• PBRT should compile now, by running make in the source directory.
• Before you can render scenes, you should set the PBRT_SEARCHPATH environment variable. In a terminal, type:
  export PBRT_SEARCHPATH = /location/to/pbrt/bin
  (the bin folder was created in the make process)

Well, I hope this is a timesaver! :)