This is an introduction to the R programming language. I would like this introduction to be as accessible and useful as possible for users of all proficiency levels. Every link below will include free resources.
Beginner Resources
Before diving into the R programming language, I would highly recommend installing the RStudio development environment. It makes a lot of interactions with the R programming language and peripheral programming related tasks (version controlling, connecting to databases, codeformatting, etc.) much easier.
If you’re a beginner and you want to get started with R as fast as possible, I recommend the following guides:

Swirl : Learn R, in R  Swirl allows newcomers to start interacting with the R programming language through guided tutorials primarily aimed at beginners.

Learn X in Y Minutes : R  Learn X in Y Minutes walks readers through an example script which demonstrates many of the important R language features without getting too bogged down on any specific topic. This is a great quick reference for the syntax of R.

R Language for Programmers  If you have any experience programming but are new to R, or would simply like a quick refresher on the basic language features of R, this is a good reference.

Cookbook for R  This website (and its corresponding book) provide examples and tutorials on many of the most common tasks that programmers are accomplishing with R.
Lastly, I want to encourage people to use the help.start()
feature in R.
Open the R console, type help.start()
, and hit enter. A window will open
which allows you to view the R manuals, check out packages’ documentation,
and search for keywords on Rspecific sites and resources.
Keep in mind that you can always use ?command
or help()
to look up help
without leaving your R or RStudio session.
More Technical Resources
Once you have some familiarity with R and want to understand it more rigorously, I think these resources can help with that:

An Introduction to R  This introduction to R is quite comprehensive and technical. I should point out that while other guides may discuss packages outside the base R language, this introduction focuses entirely on the base R language. Many people will recommend that it is important to understand the base R language, and then to quickly move onto using the nearly selfcontained ecosystems implemented in packages like the tidyverse,

aRrgh, a newcomer’s (angry) guide to R  aRrgh takes the stance that as R is a programming language predominantly developed by statisticians and not programmers, it has some very quirky and at times outright offensive features. Despite this, R comes with tools and ideas that make it uniquely suitable for statistical data analysis and after learning its quirks many people find R to be an indespensible tool they frequently reach for. aRrgh is a guide on those quirks and “gotchas.”

The R Inferno  “If you are using R and you think you’re in hell, this is a map for you.”

RBloggers : Tutorials for Learning R  RBloggers is a popular website among R users where R tutorials and blogposts are aggregated and widely distributed. This is a great post, similar to this one of mine, delineating resources in an ordering suitable for newcomers to the R language.

Efficient R Programming  This book takes a dive into the topic of writing R code which is efficient while keeping the prerequisite knowledge of the language to a minimum.

R Packages  R Packages are an integral part of the R ecosystem, and it is considered good practice to format your work as a package where possible due to the many added perks gained from R’s package development tools.

Advanced R  Advanced R takes a closer look at the technical features of the R programming language to help users improve their R programming skills.

The R Manuals  The R Manuals from CRAN are for those who want to take a deep dive into the fundamentals of the R programming language. In particular, I recommend checking out The R Language Definition and R Internals if you really want to understand how R works.
TopicSpecific Resources
While some may spend lifetimes understanding R’s internals and subtleties, others may have the goal of just getting things done. For achieving this, I would make different recommendations than I would to those who want to rigorously understand the language. To accomplish specific goals and finish your projects implemented in R, I would recommend the following resources:

RStudio’s Cheat Sheets  RStudio’s Cheat Sheets are great ataglance reference cards for many of the popular packages and features of R. They have ones for many topics, including base R, the RStudio development environment, ggplot2, a data visualization package, and many, many more.

CRAN’s Task Views  CRAN’s Task Views organize packages into topics to make them more easily discoverable.

AwesomeR  Awesome R is a curated list of R packages and tools which is great for discovering new and popular packages to help you accomplish a given task.

R for Data Science  R for Data Science, or R4DS, is a very popular book in the R community which teaches users to perform statistical analyses, visualize them, and to communicate their results effectively using the tools the R community has to offer.

The Elements of Statistical Learning: Data Mining, Inference, and Prediction is the 2nd edition of a popular, mathematically rigorous, treatment of statistical learning with R examples. The PDF of the book is available for free, and a real gift to the community.

Text Mining with R : A Tidy Approach  Another popular topic these days is natural language processing. Text Mining with R introduces ideas on how one can use the tidy data paradigm to ease their textual analyses.

Shiny from RStudio  Shiny is a web framework for R with everimproving documentation and articles. Shiny makes it easy for users to interact with data analyses through webinterfaces that functions through a reactive programming paradigm.

Show Me Shiny  In addition to the references available on RStudio’s website for Shiny, I also highly recommend the examples available at showmeshiny.com. These showcase many nontrivial and novel usecases for Shiny and can often serve as inspirational material.

htmlwidgets for R  Create JavaScript based widgets to render interactive data visualizations within R.

R Graphics Cookbook  This resource appears as if it is just one chapter inside the R Cookbook already linked above, but in actuality it is a whole book on its own dedicated to the subject of producing graphics and plots with R. This book makes heavy use of ggplot2, one of the most popular graphics frameworks which employs a theory of the grammar of graphics to make writing code to produce graphics more natural and straightforward.

Rcpp, Rcppintroduction, and the Rcpp vignettes  Rcpp is a package which provides a seamless interface between C++ and R. This is a great utility for speeding up projects in R