Neper is distributed as a free / open-source software, under the terms of the GNU General Public License (GPL). In short, this means that everyone is free to use Neper and to redistribute it on a free basis. Neper is not in the public domain; it is copyrighted and there are restrictions on its distribution (see the license and the related FAQ).
It you use Neper, please cite the Neper reference paper in your work (books, papers, talks, ...): R. Quey, P.R. Dawson and F. Barbe, Large-scale 3D random polycrystals for the finite element method: Generation, meshing and remeshing, Computer Methods in Applied Mechanics and Engineering, vol. 200, pp. 1729-1745, 2011. The paper is available for download below.
A development repository is hosted on GitHub: http://github.com/rquey/neper. It provides the development version. Code contributions to be included in the official (public) version of Neper should be submitted as pull requests.
There are two mailing lists for Neper, which you may find useful to subscribe to: