The R Project
R is a language for manipulating statistical data, and is very useful for folks in fields where this matters. It’s free software, which is important: it means that one has the freedoms to run it; to study it; to copy it; and to improve it. For those familiar with statistics, R is similar to S (which was developed at Bell Labs).
IBM developerWorks have a good Introduction to Statistical Programming with R available.
06 February 2018: updated URL