Multi Theft Auto (MTA) is an open-source software project that adds full on-line gaming support to Rockstar North's Grand Theft Auto: San Andreas PC game, in which this functionality is not originally found. It is the first open-source modification that adds a highly customizable network play element to a commercial closed-source single-player PC game. Developed as an experimental piece of software for earlier Grand Theft Auto series, Multi Theft Auto has become an advanced multiplayer platform for gamers and developers. Our software provides on-line gaming for dozens of players running user-generated gameplay content and scripts through a Lua virtual machine, and is freely distributed under the GPLv3 license.
Multi Theft Auto: San Andreas allows third-party Lua developers to include and load their own custom models and objects into the game. Here's a screenshot showing some South Park characters dynamically loaded into the game.