R is a widely used programming language and software environment for statistical computing and graphics. It is an open source freeware that comes with its own programming language, scripting capabilities, a GUI, and a lot of standard statistical methods. There are also many R packages which extend its functionality.
R Studio. Has support for version control uing SVN or git. Recommended by Graham Jones.
From the R Project:
R homepage, r-project.org.
An introduction to R, r-project.org.
Very good R bibliography in reverse chronological order, r-project.org.
Other sources:
Books:
Norman Matloff, Beginning R: The Statistical Programming Language. (Free.)
Norman Matloff, The Art of R Programming. (Not free.)
Patrick Burns, The R Inferno. (Free. What to do when you’re in “R hell.”)