SAPP (Still Another Phylogeny Program) is written by David Vance, the author of The Genealogist's Guide to Y-DNA Testing for Genetic Genealogy. It uses a combination of inputted kits with their SNP and/or STR results together with it's internal representation of the SNP tree to suggest a "best fit" tree of the kits based on a statistical model. The details of its operation are all available on the website.