iOS devices can't play many media types out of the box. At one time, you had to learn to accept that or get used to transcoding your media files - a time-consuming and tedious process at the best of times. That was until apps like AVPlayerHD came around.
This iPad video player app (iPhone version also available) lets you dump all kinds of video files onto your tablet for immediate playback. Yes, it includes top formats like MKV and Xvid, and yes you can transfer them through iTunes. Videos have to be transferred as AVPlayerHD documents, but you can wave goodbye to transcoding.
There are niggles, of course. When you stray beyond the formats supported natively by the iPad hardware - MP4, MOV - you aren't going to be able to play back 1080p video files at full speed, but decent-quality SD and 720p files should have few problems. SRT and SMI subtitles are supported too.
AVPlayerHD doesn't quite bring the iPad 2 up to the top-quality video experience offered by the Samsung Galaxy Tab 10.1, with its decent native support and drag 'n' drop file transfer, but it lessens the gap hugely.
A great way to mitigate for iOS devices' long-running lack of support for advanced video formats.