Freelancer still has a lot of online mods and servers up.
Heroes of Might and Magic 3 "Horn of the Abyss" mod has its own netplay server and built in lobby, of course you can play classic RoE/AoB/SoD H3 maps online, and it's populated. Wrath of Gods ERA3 launcher also has netplay and lobby.
And Shadzilla mentioned UT, yeah UT99 still has alive netplay community.
There could be a few living major games from back in the day still alive via community servers, I think Starcraft has some.
This is all considering you want games with servers and lobbies. If you connect with likeminded people you can play anything that supports TCP/IP.
Most of the gaming back in the day was done this way.