Plex is a popular app for Home Theater PC users that was became available last year for Windows Phone 8 and 7.x devices. The paid app connected your Windows Phone to the Plex Media Server where you can manage and stream your personal media to your phone.
A new version of the Plex app has landed in the Windows Phone Store that is a free app but only available for Windows Phone 8 Nokia Lumia devices. In just reading the Store descriptions, the paid and free version appear to be similar. However, comments in the WPCentral Forums indicate the free version resolves a few issues the paid version has and you have more options with regards to video quality.
You must have a Plex Media Server (free) installed and running on a PC, Mac or Linux system. However with the free version of Plex you will need to be running version 0.9.8 or higher of the server (paid version only requires version 0.9.6).
With the Plex Windows Phone app, you can manage your media as well as stream videos, music, photos and home movies to your Windows Phone from your home computer. Media can also be synchronized to you the cloud for easy access when your computer is offline.
You can find the free version of Plex here in the Windows Phone Store. Keep in mind it is for Nokia Lumia Windows Phone 8 devices only. The paid version of Plex (available for both Windows Phone 8 and 7.x) can be found here in the Store.
You can read more about the Plex Media Server here at the Plex website.
Update: Article fixed to reflect that this app is ONLY for Nokia Lumia phones
Thanks, Jason, for the tip!