iOS: If you're looking for a fast, solid video player for your iPhone or your iPad, Infuse can play almost anything you throw at it. It also organises your videos, TV shows and movies by automatically downloading metadata, and lets you drag-and-drop videos to your device without forcing you to use iTunes.
Infuse supports over 14 different video formats, including WMV and MKV, so you can play just about any video file you download from the web, or upload from your personal media collection to your iOS device. It also gives you an easy-to-use interface for copying those videos to your device so you don't have to push them through iTunes first -- all you have to do is open a web browser and transfer via Wi-Fi. You can also import videos from other apps, including Dropbox, Google Drive, SkyDrive and Box.
The app makes it easy to download subtitles from OpenSubtitles.org with a single tap, and can download TV show and movie cover art, ratings, and other info from other free web databases. Infuse is optimised to play even 1080p video smoothly, and supports Dolby Digital Plus mobile surround sound and AirPlay. I've been testing it for a while now, and while it's definitely a busier interface than other media players, it's also easy to use and surprisingly powerful. Infuse will set you back $5.49 in the iTunes App Store.