s&box is a modern game engine, built on Valve's Source 2
Descent 3 by Outrage Entertainment
A lightweight game server framework implemented with Actor Model
FlightGear Flight Simulator: free open-source multiplatform flight sim
Java RTS framework
The multiplayer game engine
State Management and Multiplayer Networking for Turn-Based Games
A Rust framework for building Minecraft servers
DOOM / Heretic / Hexen source port with enhanced graphics and UI
Amazon Lumberyard is a free AAA game engine
Minecraft Forge Hybrid server implementing the Spigot
3D-Accelerated open-source DOOM engine
Asphyre is a cross-platform framework for Delphi and FreePascal.
Simple meta-server for locating other multiplayer servers
Source for the Flash-based Glitch client
Tournaments and Ratings Stack
3d peer to peer chatroom where associated pairs of text gravitate