VLC is an open-source, cross-platform (available for OS X, Windows, Linux, etc.) video player (and audio player) program. Its advantage besides its cross-platform support is supporting a wide variety of video and audio codecs. That and its versatility and robust support makes it one of the most popular open-source programs available.
Apparently, VLC’s programmers have a seasonal sense of humor, as the program contains an “Easter egg” (so to speak) that automatically places a Santa Claus cap on its traffic-cone icon starting December 18, and reverts back to normal on January 1. I thought it was cute, though I suppose one’s mileage may vary.
Here’s a few screenshots of the seasonal VLC: