I really like arena shooters, especially from the iD Software heritage. One of the great things about iD is that they tend to opensource their engines after 5 years, which is great not only for the long term player communities; but provides the base of numerous 3rd party and fan projects as well.
Along with the engine code, a lot of multiplayer assets for the older games are also free to use, and servers can be run without requiring a license. Add community made mods, levels, and models and you have a deep pool of interesting games to pick from, all free.
I’ve started packaging these shooters into Docker containers so they can be set up by anyone on the quick. The first game to be containerized has been my longtime favorite: Quake II. Here’s the docker index build for drags/quake2. As usual the source is on GitHub: drags/docker-quake2
Next on the list is Warsow, followed by some iD Tech 4 derivatives: ioquake3/openarena, CPMA, Urban Terror