UCSF Chimera is described as 'highly extensible program for interactive visualization and analysis of molecular structures and related data' and is an app. There are six alternatives to UCSF Chimera for Mac, Windows, Linux and Wine. The best alternative is Jmol, which is both free and Open Source. Other great apps like UCSF Chimera are Rasmol, VMD - Visual Molecular Dynamics, ChimeraX and pymol.
RasMol is a computer program written for molecular graphics visualization intended and used primarily for the depiction and exploration of biological macromolecule structures, such as those found in the Protein Data Bank.
RasTop is a molecular visualization software adapted from the program RasMol, which was initially developed by Roger Sayle. RasTop is particularly adapted for educational purposes and for the rapid analysis of macromolecules at the bench.