All of R's functions are stored in packages. These packages are analogous to plugins for a browser. R comes with a number of default packages when installed and there are literally thousands of third party packages available for download.
Crantastic - To say that the quality & usefulness of packages varies would be an understatement. Packages can be written & contributed by anyone, so user beware. This website not only catalogues packages, but also allows users to rate & comment.
CRAN Packages - This is the official list of packages contributed to the R project. CRAN stands for "Comprehensive R Archive Network". This is the master list, although the information here says nothing about issues or bugginess.