Multi Theft Auto | Index
http://www.mtasa.com/index.php

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.

Multi Theft Auto (MTA) is an open-source software project that adds a full multiplayer network play functionality to Rockstar North's Grand Theft Auto: San Andreas (GTA:SA), in which this network play element is not originally found. It is one of the very few modifications to incorporate an extendable open-source network play element into a closed-source single-player PC game.