Half-Life singleplayer/deathmatch
https://github.com/eukara/freehl
Half-Life: Opposing Force
https://github.com/eukara/freegearbox
team fortress classic
https://github.com/eukara/freetfc
Counter-Strike 1.5
Half-Life singleplayer/deathmatch
https://github.com/eukara/freehl
Half-Life: Opposing Force
https://github.com/eukara/freegearbox
team fortress classic
https://github.com/eukara/freetfc
Counter-Strike 1.5
All of these use the great FTEQW Quake engine btw.