If you're new to R, the best place to start is the Introduction to Base R page where there are links to slides, code and data for a full-semester introductory course on R for epidemiologists as well as some screencast videos. If you have some experience with R, you may be interested in the material on using R for spatial analysis, or for the introduction to using R for simulations or perhaps web-scraping with R. I also keep notes for power and sample size calculations which make extensive use of R.