PyMOL is computer software, a molecular visualization system created by Warren Lyford DeLano.Read about PyMOL at Wikipedia.
Commercial versionFirst option is to buy the commercial version. Read the PyMOL - Academic and Non-Profit Price List. The current price at 2018 is 99 USD/year.
Open source version
The source code is available (not all features but usually good enough). You can download the source and install your self.
There are prepackaged solutions, either with prebuilt binaries or from source.
Status of PyMOL on macOS
For macOS we have tried (at 2018-03-28) the MacPorts, Homebrew and Fink distributions of PyMOL. Only PyMOL in Fink works. The MacPorts and Homebrew distributions of PyMOL are broken. This may change.
UCSF Chimera (or simply Chimera) is an extensible program for interactive visualization and analysis of molecular structures and related data, including density maps, supramolecular assemblies, sequence alignments, docking results, trajectories, and conformational ensembles.