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.