To cite the R package 'cluster' in publications use:

Maechler, M., Rousseeuw, P., Struyf, A., Hubert, M. (2005). Cluster Analysis Basics and Extensions; unpublished

Corresponding BibTeX entry:

  @unpublished{,
    title = {Cluster Analysis Basics and Extensions},
    author = {Martin Maechler and Peter Rousseeuw and Anja Struyf and
      Mia Hubert},
    year = {2005},
    note = {Rousseeuw et al provided the S original which has been
      ported to R by Kurt Hornik and has since been enhanced by Martin
      Maechler: speed improvements, silhouette() functionality, bug
      fixes, etc.  See the 'Changelog' file (in the package source)},
  }