VLC used to be the premo app for anyone who wanted to play any sort of video (movies, tv shows, etc.) on their iPad. Sadly, it was pulled from the app store. Luckily, there's It's Playing. It's like VLC except you can download it RIGHT NOW.
It's a video player app for your iPhone and iPad. And like VLC before it, it plays every video file from here to the sun: AVI, MKV, WMV, OGG, FLV and more. What's great is that you don't have to bother converting your video files before you put it on your iPad (like you do with the stock video player), you just plug in your iPad or iPhone to your computer, drag and drop your movie file (that you might've procured from dark areas of the internet) into the It's Playing app (under Apps Tab > File Sharing) and then watch! Seriously, it'll transfer the movie straight to the app in no time and you can watch on your iPad right away. No fuss!
Yeah, if you were an un-lazy human being you could fire up HandBrake on your computer and then convert the files to a file type that Apple doesn't hate but MY GOD THAT TAKES A LOT OF WORK. It's Playing takes all those stupid steps out of the way. Also, It's Playing lets you tweak the video file by adjusting contrast, color, brightness and even upping the volume on the fly. Those are biggish small tweaks that can turn an unwatchable transfer into something worth sitting down for. The non-standard video controls are admittedly a little weird (you'll ask yourself what that weird Gemini looking Euro/Pound logo looks like). There's also gestures that let you skip around the movie and options to re-name, cut and privatize particular files. For those who missed the boat on VLC, It's Playing a is a decent alternative.