Given a fully-reduced emission line spectrum, fivel will compute the temperature and density of the emitting gas. For an idea of which spectral lines make good "thermometers" or "density indicators", look here!
When you're ready to download the program, click here.
To run the program after compiling the code, type 'fivel' and follow the directions. Upon quitting, an output file containing the results (called 'fivelout.dat') will be produced.