R's biggest strengths is its unparalleled graphing capabilities. Just see any of our previous posts on ggplot2, visualization, or other posts tagged with R. R has several fundamentally different systems for plotting, including base graphics, lattice, and ggplot2. Furthermore, many add-on packages come with their own functions for producing problem-domain specific graphics. For example, see GenABEL, a very nice R package for GWAS analysis, which has functions for producing manhattan plots, LD plots, etc.
Now let's say you've seen a certain graphic before, and you want to find the package you need to download and which function you should use to make the plot. That's where the R Graph Gallery and the R Graphical Manual can become very useful. Both sites give you thumbnail previews of graphics produced by functions bundled with certain R packages, code for producing the graphic, and which R packages you need to download for the functions used to create the graphic. The R Graphical Manual is much more comprehensive, and is categorized based on CRAN Task Views (CTV) categories (check out all 29 pages of graphics in the Genetics task view).
R Graphical Manual
R Graph Gallery