It's fair to say that Hadley Wickham, chief scientist at RStudio and a new member of the R Foundation, has made great contributions to the R community. Not only is he the author of several R-related books including Advanced R, Hadley is also the author of dozens of R packages which have transformed the way that data scientists work with R. His packages include an entirely new graphics system (actually, two), a powerful language for manipulating data, tools for importing data files, and developer tools for other R package authors that have influenced R packages way beyond his own.
If you use R it's extremely likely you've used at least some of Hadley's packages. For everything else, check out The Hitchhiker's Guide to the Hadleyverse, Adolfo Álvarez's comprehensive guide to all 55 packages with Hadley's contributions (to date). Or for an overview of the packages in slide form, take a look at Barry Rowlingson's presentation, "Packages of the Hadleyverse: Power for your R".
Thanks Hadley for all your contributions!