Team Fortress 2

Valve shipped this class-based shooter in 2007 inside The Orange Box, went free-to-play in 2011, and has quietly maintained it with a shrinking crew ever since. Nine classes, each with one clear job and no loadout that lets you cover for a teammate: Heavy is a wall of bullets that cannot function without a Medic, Medic is a support class whose charge heals through anything for eight seconds, Engineer builds sentries and teleporters, Spy disguises himself as you. The cartoon art direction and the Meet the Team shorts gave it a personality that outlasted its own graphics, and those shorts are still the best character introductions the genre has. It also, accidentally, invented the modern cosmetic economy: hats became tradeable items, items became a market, and every crate-and-key system in the industry traces back here. Nearly two decades on the servers are still busy, the community still runs its own maps and modes, and it has become a case study in how long a well-designed multiplayer shooter can survive.