General R References
R Reference Card: 4 pages that include 90% of the R commands you will ever need.
ESS Reference Card for S and R: Reference card for Emacs Speaks Statistics - a plugin that allows R to run in the Emacs text editor (available on the RCE).
RSeek: A search engine just for R.
Google's R Style Guide: Basic rules of coding in R
Quick-R: Great introduction to R, as well as reference for advanced R users. Covers data input, data management, basic and advanced statistics, and graphics in R.
References to Useful R Packages
Zelig: A useful package for running all types of statistical tests.