PLUMED is free and licensed under the Lesser GNU General Public License. We only ask you to cite the following references in any work you publish using PLUMED:
If you used PLUMED 2.x: G.A. Tribello, M. Bonomi, D. Branduardi, C. Camilloni, G. Bussi, PLUMED2: New feathers for an old bird, Comp. Phys. Comm. 185, 604 (2014), preprint available as arXiv:1310.0980
If you used PLUMED 1.x: M. Bonomi, D. Branduardi, G. Bussi, C. Camilloni, D. Provasi, P. Raiteri, D. Donadio, F. Marinelli, F. Pietrucci, R.A. Broglia and M. Parrinello, PLUMED: a portable plugin for free energy calculations with molecular dynamics, Comp. Phys. Comm. 180, 1961 (2009), preprint available as arXiv:0902.0874
The development version is hosted on a public git repository. Feel free to try it and provide feedbacks!