Multiplayer frameworks and game servers.

Software	Description	License	Tags	Best For
Epic Online Services	Free multiplayer, matchmaking, voice chat. See also: Software Reference → Pipeline & Production Management Software	Free	Free EOS · Matchmaking/Voice	Free multiplayer services
Nakama	Open-source game server (social, matchmaking, leaderboards). See also: Software Reference → Pipeline & Production Management Software	Open Source	Social Server · Matchmaking	Social game server
Photon Engine	Cloud multiplayer (PUN, Fusion, Quantum SDKs). See also: Software Reference → Pipeline & Production Management Software	Freemium	Cloud Multiplayer · Unity/Unreal	Cloud multiplayer
SmartFoxServer 2X	A SDK for rapidly developing multiplayer games and applications with Adobe Flash/Flex/Air, Unity, HTML5, iOS, Windows Phone 8, Android, Java, Windows 8, C++. See also: Software Reference → Pipeline & Production Management Software	Freemium

Related:

Basic Multiplayer Unity. Fast-Paced UDP Client-Server implementation.
Bomber. Open-source Bomberman multiplayer game made in Unreal Engine 5.
Colyseus. Node.js multiplayer framework.
Colyseus Unreal. Colyseus Multiplayer SDK for Unreal Engine
Fish Net. High-performance Unity networking.
Forge Networking Remastered. Free open-source multiplayer networking system with good Unity integration.
Gambetta_Networked Demo. Fast-Paced Multiplayer: Sample Code and Live Demo - Gabriel Gambetta's Multiplayer Network Demo in Unity C# as Networked using Lidgren Network.
GameDev.tv Unreal Multiplayer Master: Video Game Dev In C++ Course Paid. Course on building Unreal Engine multiplayer games in C++.
Mirror (GitHub). A community replacement for Unity's abandoned UNET Networking System.
Mirror (site). Open-source Unity networking.
Unity Multiplayer Packages. Unity multiplayer packages and samples.