Free downloads are available for Linux, Mac OS X, and Windows. Setup procedures vary depending on the system you are using. Please see the “Manuals” link on the left hand side of the download screen if you experience any problems.
R Short Reference Card
Extensive, while not exhaustive, list of commands for R.
Free downloads of the software for a variety of operating systems, and many examples to play with.
Installing igraph in "R"
Prof. Jordan Ellenberg's how-to page, plus links to data sets and helpful articles.