Dan's R Cheat Sheet: a 2-page summary of the functions I use the most.
Cheatsheets by RStudio: info-packed 2-page guides for graphing, data wrangling, markdown, etc.
Quick-R: an excellent searchable reference guide
Cookbook for R: cookbook-style how-to wiki, particularly useful for experimental psychology
Penn R study group: detailed reference in the form of tutorial notes
ggplot2: documentation website for our main graphics tool
A great ggplot2 workshop.
A chart chooser: simple examples of many different kinds of ggplot graphs.
The R graph gallery: many different graph examples with snippets of code for generating them.
Markdown Cheatsheet: a quick reference for markdown syntax.
Matlab/R Reference guide: A handy reference for both R and Matlab (our other main analytical tool)
Hadley Wickham's Style Guide
R Bloggers: A blog aggregator pulling together posts from over 140 bloggers who write about R in English
StackOverflow - R: A great community Q & A site. If you have a problem, they can usually help you.
Applied R for the quantitative social scientist by Rense Nieuwenhuis (free pdf!)
R in a Nutshell by Joseph Adler
R for SAS and SPSS Users by Robert Muenchen
Analyzing Linguistic Data by Harald Baayen
ggplot2: part of the Use R! series of focused guide books
Books that teach statistics and R together:
Learning Statistics with R by Daniel Navarro (ebook is free!)
Discovering Statistics Using R by Andy Field, Jeremy Miles, & Zoe Field