R is a language and environment for statistical computing and graphics. It is a GNU project which is similar to the S language and environment which was developed at Bell Laboratories (formerly AT&T, now Lucent Technologies) by John Chambers and colleagues. R can be considered as a different implementation of S. There are some important differences, but much code written for S runs unaltered under R. - The R Project website.
R is Freeware, and is DOD Complient on a number of platforms.
R has packages developed for just about everything imaginable, including creating your own xkcd-style graphics.