It catalogs and let's you play/transcode to devices. It's a replacement for Plex based off a fork of Emby; however, Jellyfin is completely open source.

I just installed it a few weeks back, and like it a lot. Much less clutter and overhead compared to Plex. The Roku app is what's missing for me, though. When that is finished (it's in development), this will be a great full replacement for Plex, for me.