"Articles on Statistical Computing - R" from the Social Science Computing Cooperative, at the University of Wisconsin-Madison
"Cookbook for R," by Winston Chang
"Getting Started in Data Analysis Using Stata and R," by Oscar Torres-Reyna, Princeton University
"Getting Started in R~Stata: Notes on Exploring Data," by Oscar Torres-Reyna, Princeton University
"Introducing R," by Germán Rodríguez, Princeton University
"Quick-R - 'Accessing the Power of R'"
"r4stats.com," by Robert A. Muenchen, the University of Tennessee
"RStudio Cheat Sheets"
"R for SAS and SPSS Users," by Robert A. Muenchen, the University of Tennessee (.pdf)
"R for Stata Users," by Matthieu Gomez , Princeton University
"Resources to Help You Learn and Use R," from UCLA's Academic Technology Services
"Stata Fully Mapped into R," an R Package for Translating Stata Syntax into R Syntax
"Using R and RStudio for Data Management, Statistical Analysis, and Graphics" (Second Edition) (.pdf)