mtasa-blue

Grand Theft Auto: San Andreas shipped in 2004 with no multiplayer at all. Multi Theft Auto (MTA) is the engine that makes that possible. It's a multiplayer mod framework that turns a 2004 single-player game into a platform for custom online experiences: racing, roleplay, zombie survival, whatever the server community builds.

GPL v3, C++ core with Lua scripting for server-side game logic. The project has been active since the mid-2000s and still maintains an active player community. Servers run custom game modes written in Lua, and the modding API is extensive. You can alter nearly every game mechanic.

Fully free. The mod client and server are free. You need a legitimate copy of GTA: San Andreas to play. Server hosting: run it on any machine or VPS. A small VPS ($5-10/mo) handles a decent player count. The Lua scripting API is well-documented and has a large library of community resources.

The catch: this is a niche community project for a 20+ year old game. If you're not already in the GTA modding scene, this probably isn't for you. The codebase is old and complex. But for its community, MTA remains the most feature-rich multiplayer mod for San Andreas, beating the competing SA-MP (San Andreas Multiplayer) in scripting capabilities.