Select the version of the code you want, click the "Download Now" button, and your browser should download a gzipped tar file. Unpack it with the following commands, and look for a README to get you started.
gunzip file.tar.gz tar xvf file.tar
Note that the C++ version of LAMMPS, will unpack into a directory like lammps-25Mar09. If the date is later than the release date listed below, it means you have downloaded an upgraded version that includes the bug fixes and new features listed on this page up to and including the date of the directory. However, when LAMMPS runs it will still print out the release date listed below to the screen, since that is not changed every time there is a bug fix.
All versions of LAMMPS are distributed under the terms of the GNU Public license. The earlier Fortran versions are no longer under active development.
There have been ~39,600 downloads of LAMMPS since Sept 2004.
Older parallel MD codes (ParaDyn, LJ) and the Python-based Pizza.py package, which provides pre- and post-processing and viz tools for LAMMPS, are available for download from this page.