In addition to the manuals, FAQs, the R Journal and its predecessor R News, the following sites may be of interest to R users:
R guides and documentation not contained in the contributed documentation section of CRAN:
Currently none are freely available.
Disclaimer: Most of the documents listed on this page are provided by users of R. The R core team does not take any responsibility for contents (except when explicitly named as author), but we appreciate every effort to create R-related documentation very much and encourage everybody to contribute to this list!