R is a language and environment for statistical computing and graphics.
R is available as Free Software. You can download R using this link
RStudio is a separate piece of software that works with R to make it much more user friendly and also adds some helpful features.
You can download R Studio here:
The UCLA Academic Technology Services R resources page contains tutorials and useful resources to learn R
Germán Rodriguez from Princeton University, provides useful notes how to get started with R and how to fit linear and generalized linear models
R-bloggers. You will find posts, news and tutorials on R.
Go to http://patilv.github.io/ if you want to create graphs and charts using R. It has some sample codes to create cool visualization projects
R: A self-learn tutorial. Introduction from National Center for Ecological Analysis and Synthesis at UC Santa Barbara
Statistics with R Computing and Graphics. You will find a 15-page introduction to R . By Kjell Konis, then at the University of Oxford
Little Book of R for Time Series. This is an useful resource to use R for analyzing data collected over time. By Avril Coghlan at the Wellcome Trust Sanger Instituie, Cambridge, U.K.
Introduction to ggplot2. 11-page introduction to ggplot2, by N. Matloff at UC Davis.