plyr is a set of tools that solves a common set of problems: you
need to break a big problem down into manageable pieces, operate
on each pieces and then put all the pieces back together. For example,
you might want to fit a model to each spatial location or time point
in your study, summarise data by panels or collapse high-dimensional
arrays to simpler summary statistics. The development of plyr has
been generously supported by BD (Becton Dickinson).

Homepage:
https://CRAN.R-project.org/package=plyr
