Multi Theft Auto
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 initially for Grand Theft Auto 3 back in early 2003, 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.
Current stable release: MTA:SA 1.3.1 (released on 2 September, 2012)
This is an official Multi Theft Auto group on Steam Community.
Note: MTA:SA works fine with the Steam version of GTA:SA, albeit it needs some small tinkering. Please check out one of the attached links to see what needs to be done.