The History of Sound Card Spectrometry
On this page it is worth mentioning a little about the history of Sound Card Spectrometry, how it was developed in Australia by Marek Dolleiser and how it evolved.
Marek works at The University of Sydney, with third year physics students, and developed PRA as an educational tool for his students. His novel pulse shape algorithms and software performed so well, he decided to offer it to everyone for free. After the Fukushima nuclear disaster the software became popular in Japan and spread to the rest of the world like wild fire. This in turn inspired many other software writers, who also added their own versions of free sound card spectrometry software. To this day, the software is being improved, and there is healthy competition between software authors, who can produce the best version.
Take your pick below, PRA, BecqMoni, Theremino, GeigerBot, Fitzpeaks, all work well and have their own features and benefits. I do recommend you start with PRA which was the original and still in my opinion the one that consistently performs to scientific standards.