Group for all old school FPS fans.

Sven Coop for the original Half-Life is a free standalone title on steam now
https://steamproxy.net/steamstore/app/225840/Sven_Coop/

Action Quake 2 made its way to steam as AQtion
https://steamproxy.net/steamstore/app/1978800/AQtion/

The classic build engine shooter Shadow Warrior has been free on steam since forever, this is only the emulated dos version but the data files can be used with any Shadow Warrior source port.
https://steamproxy.net/steamstore/app/238070/Shadow_Warrior_Classic_1997/

The Marathon games are available on steam. A mac series from Bungie most of us PC gamers missed out on.
https://steamproxy.net/steamstore/app/2398450/Classic_Marathon/
https://steamproxy.net/steamstore/app/2398490/Classic_Marathon_2/
https://steamproxy.net/steamstore/app/2398520/Classic_Marathon_Infinity/