Hi measurement freaks,
here is a site with free software, not all is useful tho.
http://www.pvconsultants.com/audio/frdgroup.htm

I am using 'SPL Tracer' to graphically read in frequency response
measurements. 'Linkwitz Transform' lets you design a filter to correct
closed box low end to any desired frequency response.
Hope you guys have fun.
--
ciao Ban
Bordighera, Italy