What it does: R is a general statistical analysis platform (the authors call it an "environment") that runs on the command line. Need to find means, medians, standard deviations, correlations? R can handle that and much more, including "linear and generalized linear models, nonlinear regression models, time series analysis, classical parametric and nonparametric tests, clustering and smoothing," according to the project website.
Check out the article linked below for the full write-up on R and information about lots of other free and useful tools for data scientists.
ComputerWorld: 22 free tools for data visualization and analysis