Hadley Wickhams's GitHub page for his advanced R development courses. Includes useful material on advanced programming in R as well as package development.
Handy info on branching and merging with Git.
It's exactly what it sounds like. Good? Slightly horrifying?
Markdown package for the R statistical software environment. Handy for publishing R code (e.g. to Rpubs) and for integrating latex, comments and other text more seamlessly into code.