Plex Web External Player
Chrome doesn't come close to the picture quality of that provided by madvr based players such as MPDN or MPC-HC. This is a modification that adds another play button to the web interface which launches your selected media player with the item you selected to play in Plex. It'll also make it as viewed but has the limitation that it can't seek or set partial views. Its tested and built for Windows and Chrome but might work elsewhere.