R is also a statistical
programming language, indirectly derived from
S+. It is referred to as
GNU/S by those who contribute to it, and it is released under the
GPL.
As is S+, R is a very robust, and not at all user friendly, statistical platform. The reason that it is not at all user friendly is in order to allow users the maximum flexibility in constructing their models. At present, there are very few statistical computations that cannot be performed by the R system.
Given that it is a relatively new project (started in 1995, if I'm not mistaken), it is currently under very heavy development, but uses a modular architechture to allow researchers to contribute new algorithms seamlessly into the program. It works under all *NIX variants, Windows 9x, Macintosh and there are some early releases for Mac OS X.
For further information, or even to take it for a test drive, please visit http://www.r-project.org/.