What documentation exists for R?

Chapter 3: R and S 19
3.4 Is there anything R can do that S-Plus cannot?
Since almost anything you can do in R has source code that you could port to S-Plus with
little effort there will never be much you can do in R that you couldn’t do in S-Plus if you
wanted to. (Note that using lexical scoping may simplify matters considerably, though.)
R offers several graphics features that S-Plus does not, such as finer handling of line
types, more convenient color handling (via palettes), gamma correction for color, and, most
importantly, mathematical annotation in plot texts, via input expressions reminiscent of
TEX constructs. See the help page for plotmath, which features an impressive on-line
example. More details can be found in Paul Murrell and Ross Ihaka (2000), “An Approach
to Providing Mathematical Annotation in Plots”, Journal of Computational and Graphical
Statistics, 9, 582–599.
3.5 What is R-plus?
For a very long time, there was no such thing.
XLSolutions Corporation is currently beta testing a commercially supported version of
R named R+ (read R plus).
In addition, REvolution Computing has released RPro, an enterprise-class statistical
analysis system based on R, suitable for deployment in professional, commercial and regulated
environments.