If you wanted to use the Plex Media Server inside Kodi, and get the best of both services in the process, you used to need a Plex Plus subscription. Now, though, that's changed, because Plex's Kodi add-on is freely available to everyone.
Here's how Plex describes the add-on:
A dedicated device running Kodi and the Plex for Kodi add-on gives you a slick, configurable way to play your media in your home theatre. Kodi AV settings enable powerfully advanced knob twisting and lever sliding to fine tune playback for a customised viewing experience! Settings nirvana!
Some features are still unavailable if you don't have a subscription, like audio fingerprinting and parental controls, but the fact of the matter remains the same. If you like Plex, but can't ditch everything that makes Kodi great, you don't need to pay for the privilege anymore.
Surprisingly this news comes less than a week after it was revealed Plex had taken legal action against unofficial add-ons that let people integrate the two services without paying.
Having the two together is a bit weird, though, since Plex did begin life as an add-on for Kodi back when it was still known as XBMC. Still both have their respective advantages, and now it means you don't need to pay extra because you don't want to keep switching between apps.