Sven Co-op

Sven Co-op is free on Steam and has native server support that dates back to when LAN parties were the only way to play multiplayer. That combination makes it an easy install for groups where not everyone wants to spend money on something unfamiliar. The Half-Life campaigns and the massive library of community maps give you enough content to fill multiple sessions, and the classic GoldSrc feel is either charming or offputting depending on your age. I find it charming. The accessibility score reflects that newer players will need a few minutes to adjust to the movement and pacing. Worth it if your group has even a passing interest in where PC co-op shooters came from.