devtoolspackage by Hadley Wickham, Jim Hester, and Winston Chang, with functions to simplify the tasks when developing R packages.
usethispackage by Hadley Wickham and Jenny Bryan, with handy workflow functions for R packages and other R-based projects.
roxygen2package by Hadley Wickham, Peter Danenberg, Gábor Csárdi, and Manuel Eugster for writing documentation.
pkgdownpackage by Hadley Wickham and Jay Hesselberth, creates nice-looking websites based off the documentation for R packages.
testthatpackage by Hadley Wickham, simplifies the process of writing and running tests.