R has a lot of functions. Add in all the packages on CRAN, and then there's a lot of functions. You can search through the contents of the function documentation at places like Rseek.org, but the R Graphical Manual takes a different tack on exploring the functions in R: rather than browsing by the name or description of the functions, you browse by the graphics generated by the functions.
One of the very clever design decisions by the R Core group was to enforce that every function should include documentation, and that the documentation should include a working example. Those examples actually act as unit tests -- part of the process of testing R and R packages is validating that the example code actually runs without error.
Because graphics are such an integral part of the R language, many such examples include code that generates graphs. As far as I can tell (although it's not altogether clear), the site works by running the example code included with each function and saving any graphs they generate. You can then browse through all the graphs. Click on one that interests you to see the function and the example code that generated it. You can also browse through graphics from particular domains (Pharmacokinetics or Finance, for example), as defined by the various CRAN Task Views.
R Graphical Manual: Index