PSPP

The most powerful free and open source (FOSS) statistics program, though, is R. Originally a FOSS version of the statistics language S, R has shown explosive growth over the last few years, with some 7,000 add-on packages available to handle nearly any statistical requirement and an increasing number of books, courses, and blogs (e.g. R-bloggers) focusing on practical usage. Some websites concentrate specifically on how to use R for psychological research—an example is William Revelle's Personality Project, which also offers an R package called psych, a toolbox for personality, psychometrics, and experimental psychology...