Multi Theft Auto adds networked multi-player to the Windows versions of Grand Theft Auto 3 and Grand Theft Auto: Vice City. This ingenious “giant hack” illustrates just how far a commercial game can be extended beyond a typical “mod,” even if one does not have access to the original source code. The developers sum up what it took to create their unofficial mod: open source software (RakNet and Crazy Eddie’s GUI System), along with lots of time, persistence and junk food.
link to download MTA : http://www.mtavc.com/
What a fairly ingenious way of hacking multiplayer into a game without having access to the original source code. Kudos to them.