iOS: Today, the Plex app for iOS gets a big update with a bunch of new features and a new free tier that provides basic functionality.
With the free version, you can use the app as a remote or to play the contents of your camera roll. To get everything else, you'll need to upgrade via an in-app purchase ($6.49, but it's included if you already purchased the app). The app also gets a design overhaul to improve basic functionality, adds support for video chapters, a new audio players, a new rich media browsing system, and Rotten Tomatoes ratings. Mobile sync is improved as well, so it's more reliable and a bit faster.
Plex (Free/$6.49) [iTunes App Store]