VLC was an iOS app in the App Store for a brief period of time, until one of the code contributors asked Apple to pull it down.

http://www.videolan.org/vlc/download-ios.html still has the binary, or the source to compile yourself. To install on a non Jailbroken device, you will need an Apple Developer Certificate, via their $99 developer program. VLC 2.0 source isn't iOS compatible, but older versions were being kept up to date, and work I believe is still progressing on 2.0. I don't follow it too closely though.

It was pretty harsh on battery life, as it used software decoding, but it worked okay. I still have the App Store version around somewhere.