This is the official page of the free program BHS. It is a simple Python script, and I am not a Python expert, so don't expect much from it.
Since BHS is free software, you can download, use, modify and redistribute it to your heart's content, with some caveats (see license). If you would like to share your corrections/additions with me, I'd gladly hear of them by e-mail.
Basically BHS downloads a host.gz from the stats directory of a given BOINC project, and processes it to extract what percent of the total computers that contribute to that project (and the credit they yield) corresponds to different OSs (Windows, GNU/Linux, Mac OSX and others).
I have written some posts about the evolution of this script in my blog: