Undoubtedly, one of the most popular open source media players available today is VLC, and it's been that way for quite some time now. As a project backed by many well versed contributors it's available in a wide range of platforms: Windows, Mac, Linux, Android and now once again iOS.
Being an open source project, as long as you oblige the GNU license agreement, it's perfectly fine to distribute and port to practically any environment. Based on that premise Applidium posted a port of VLC on the App Sore a couple of years back, but due to an incompatibility of licensing terms it was ultimately removed by Apple. Fortunately, for the millions of us who use software and are interested in taking its functionality on the go, VLC is back on the App Store, this time officially backed by the VideoLAN team.
The new version 2.0.1 can handle all major video formats including MKVs, as well as audio tracks and subtitle tracks. It also includes support for network streams like HLS and MMS, Bluetooth headsets, AirPlay and Dropbox integration. For more details on features and requirements, as well as the download link for the App Store and Jailbroken devices running iOS 5.1 or later, just click here.