I run a plex media server on my 3b+. The server is on the wall below my router (attached via ethernet), a 2TB HDD sits next to the server. I put movies & TV for my kids on the server, then they can watch on one of two TVs in the house via Roku. It's good for getting them off trashy Netflix movies & making classics (or what I consider classics at least) available to them.

I chose this configuration rather than running plex in the cloud because a) don't want to pay monthly forever for something I use a few times a week; b) less wasting power and c) this at least theoretically can work during an internet outage (though plex authentication may make this difficult). I configured the HDD to spin down after 20 seconds of no r/w, so the whole thing draws very little power while idling (or so I assume).

The major limitation of this setup is that the pi cannot handle video transcoding. As long as I transcode to something the Roku supports natively this isn't an issue: transcode once (on my laptop), put it on the Pi, play whenever. I have yet to script this process but that's my next step in the project.

It will probably be a year before I realize any cost savings (a friend pays CAD20/mo for a hosted setup which also handles on-the-fly transcoding), but, well, it's a simple server and I just wanted to do it myself, gosh darn it!