Further reading

Books and tutorials

Most books on R are about ‘how to do statistics in R, for someone who knows statistics but doesn't know programming’. They are rather challenging if you know programming but don't know statistics. Here is a small selection of titles.
Modern applied statistics in S, W.N.Venables and B.D.Ripley [amazon]. Comprehensive and terse.
R Graphics, Paul Murrell [amazon]

Forums and galleries

Public datasets