Let's see. From the time of CS......... Almost any game with multiplayer mode.
We used to have monthly LAN games back at our old house (before we had broadband) and still do at relatives' houses. We'd use regular wired, WiFi, and even Firewire cables to hook into the home network.
As for the games. Since many of us are flight simmers... My own favorites:
MS Combat Flight Simulator 2
Homeworld
Battlefield 1942 with Desert Combat Extended (DCX) mod
Battlefield 2
IL-2
Homeworld 2
Pacific Fighters
And yep. We still have Quake 2 so that those kids with prehistoric Pentium pc hand-me-downs can join in.
We had Far Cry as well at one time. But, it seemed we spent more time looking for each other than actually fragging.