R for Beginners by Emmanuel Paradis. An introduction to R (pdf: 72 pages) .
R Reference Card by Tom Short. A list of R-commands with brief descriptions (pdf).
Using R for Data Analysis and Graphics - Introduction, Examples and Commentary by John Maindonald (PDF, data sets and scripts are available at JM's homepage).
Ekologisk statistik med R! A webpage with brief descriptions and examples of statistical analyses using R. Compiled by Didrik Vanhoenacker, Dept. of Botany, Stockholm University (Swedish).
Pimp your Graph by Didrik Vanhoenacker. Help file for making nice graphs.
Lazy Dog by Patrik Lindenfors. Code for common R tasksused during the course.