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).

WWW: http://cran.r-project.org/web/packages/plyr/