Xonotic

Free, open-source, runs on hardware from 2012 without complaint. Xonotic is the answer when someone at your LAN forgot to bring a gaming machine and is on a work laptop with integrated graphics. I've tested arena shooters on modest hardware repeatedly for exactly this reason and Xonotic holds up in a way that most of its genre peers don't. The movement is fast, the weapons have personality, and the server browser makes private match setup trivially easy. It's not going to replace Quake Live for arena purists or TF2 for mixed groups, but as a zero-cost fallback that actually plays well, it's invaluable for a LAN host who wants options.

If Teeworlds is the lightweight filler option and Team Fortress 2 is the structured team game, Xonotic is the one you put on when the group wants pure arena shooting and does not want to argue about classes or objectives. Movement is fast, weapons feel deliberate, and the server setup for local play is well documented. I know this type of game is not for everyone, the Quake DNA is strong and uncompromising, but for groups where at least half the room grew up on arena shooters, Xonotic hits something nothing else on this list quite does. Completely free, open-source, and the kind of game where someone is inevitably still playing at 2 AM.