Colyseus

Photon

photonengine.com

Popular

Exit Games' hosted multiplayer SDKs (Fusion, Quantum, PUN), dominant in Unity games.

Steamworks

partner.steamgames.com

Popular

Valve's SDK and services for Steam: matchmaking, P2P networking, achievements, cloud saves and the store.

Netcode for GameObjects

Unity-Technologies/com.unity.netcode.gameobjects

Unity's official high-level networking library for GameObject-based multiplayer.

Mirror

MirrorNetworking/Mirror

Open-source high-level networking library for Unity, evolved from UNET.

FishNet

FirstGearGames/FishNet

Free, feature-rich Unity networking with client-side prediction and lag compensation.

Nakama

heroiclabs/nakama

Open-source game server for accounts, matchmaking, chat, leaderboards and realtime multiplayer.

Epic Online Services

dev.epicgames.com

Free cross-platform services from Epic: accounts, lobbies, voice, matchmaking and anti-cheat.

Agones

agones-dev/agones

Kubernetes-based open-source platform for hosting and scaling dedicated game servers.