Roll20 - Roll20 is a suite of easy-to-use digital tools that expand pen-and-paper gameplay.
GOG.com - DRM-free game store, selling both new and old titles. No clients required.
Astral Tabletop - Create, host, and manage your tabletop role-playing games — online or in person
Diep.io - Diep.io is a multiplayer action game available for web browsers, Android, and iOS, created by Brazilian developer Matheus Valadares. Players control tanks and earn points by destroying shapes and killing other players in a 2D arena.
D&D Beyond - Official online toolset for Dungeons & Dragons tabletop gaming
Chess.com - Play chess on Chess.