Synergy is a free mod for Half-Life 2 that allows you to play the Half-Life 2 series and many other singleplayer Source Mods cooperatively with friends. It was recently upgraded to support the Half-Life 2 20th Anniversary Update.

Read the full article here: https://www.gamingonlinux.com/2024/12/half-life-2-co-op-mod-synergy-upgraded-for-the-half-life-2-20th-anniversary-update