Arguably the most popular piece of open-source software after Firefox, VLC or VideoLAN Client, made its comeback to iOS after a 2-year sabbatical. VLC left the iOS platform after its license was found to be incompatible with Apple's platform. The new VLC 2.0 for iOS comes with support for all formats as its desktop (PC) variant, and a license that doesn't step on Apple's toes.
The new VLC for iOS even brings a few additions, for starters, you get support for AirPlay, network streams (Internet radio/video), background audio, and interoperability with Dropbox; not to mention support for Retina resolutions, and support for all the new iDevices launched in the past two years. It's available on the App Store now; while its source code will be released a little later today. Apparently Apple couldn't stop it from being open-source.
DOWNLOAD: VLC for iOS