To cite plyr in publications use:

Hadley Wickham (2011). The Split-Apply-Combine Strategy for Data Analysis. Journal of Statistical Software, 40(1), 1-29. URL http://www.jstatsoft.org/v40/i01/.

Corresponding BibTeX entry:

  @Article{,
    title = {The Split-Apply-Combine Strategy for Data Analysis},
    author = {Hadley Wickham},
    journal = {Journal of Statistical Software},
    year = {2011},
    volume = {40},
    number = {1},
    pages = {1--29},
    url = {http://www.jstatsoft.org/v40/i01/},
  }