VMD

From wiki.gromacs.org

Visual Molecular Dynamics (VMD) is a superb, professional-quality award-winning molecule visualization package freely available for a wide variety of platforms. Full manuals and tutorial material are available from that link.

It can read a variety of GROMACS-compatible file formats, including .pdb, .gro, .g96, .xtc, and .trr, as well as many others. It cannot read .top or .tpr files.