Short bio: Computer Scientist, FOSS supporter (read more)
Tux Machines (TM)-specific
Gwenview is the default image viewer for the KDE desktop. Out of the box it's good looking but nothing obviously exceptional. Except for two things: Phonon and plugins. That combination really does make Gwenview a pretty useful bit of kit.
Phonon is the KDE technology that offers a consistent API to use audio or video within multimedia applications. Gwenview qualifies under that description. Fire up Gwenview and navigate to the directory where you store your videos. Simply click on one and Gwenview will start playing it. It's that simple. Don't believe me? Here's a screenshot of the blessed James Taylor in full concert flow, with Gwenview handling the mp4 file with seamless ease.