VLC, by the VideoLAN Organization, is a media player. It plays audio and video, but what makes it special is the amount of formats that it can cope with. Many times I have tried to play something and the built-in media players have failed to perform, sending me off on often fruitless searches for this or that codec. Apart from broken media files, VLC has always been able to play them.
VLC is also great for converting, those awkward unknown formats, to standard formats. I have used it to convert video files to formats all computers can play, and audio files. With audio files, and with various sites restricting the size of said files, VLC comes into its own. The ability to change bitrate during conversion is really helpful. Put simply, if you need a smaller file, VLC can help.
So if you have a need to play unruly media files, or need to convert them to an alternative format or size, then VLC is the best app I have found. Download the free app from the following link, http://www.videolan.org/vlc/index.html