"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
"Quick-R - 'Accessing the Power of R'"
"R - "Quick List of Useful R Packages"
"r4stats.com," by Robert A. Muenchen, the University of Tennessee
"R Software - Easy Guides," from Statistical Tools for High-Throughput Data Analysis [NOTE: this site is a bit dense; scroll down for R tutorials.]
"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)
"Using R to Analyse Key UK Surveys," from the UK Data Service (.pdf)