Chimera
Chimera and ChimeraX are very popular among structural biologists for visualization of atomic models and density maps. Chimera is free for non-commercial use, and requires a license for commercial use.
Chimera is easier to use than PyMOL. However, FirstGlance in Jmol is much easier than Chimera, provides a great deal more help and background information, and has many unique capabilities and shortcuts absent in ChimeraX. Each program has its strengths and weaknesses.