ALCHELEMENTAL,
My new 2 player strategy game of Alchemical Elemental Geometries,
currently in Print-and-Play (PDF) format and as a Vassal module. Click
here for info and links or click here for the module at vassalengine.org

itch.io has many indie games

cardgames.io for solo Hearts and other card games

boardgamearena.com has many games including multiplayer

happymeeple.com is great for beginning gamers

playdiplomacy.com for online Diplomacy

gamesgames.com has assorted games

kongregate.com also has a wide assortment

thegeneralshq.org for real human hexmap wargames

Google search for "online free games"