My dreams have come true. Some clever people out there have made a hack for GTA called Multi-Theft Auto. It allows people to play in the GTA city in a multiplayer environment. As /. notes:
Now I can streetrace and play Liberty City Survivor with real people, not just use the “crazy pedestrians” cheat. Rockstar would be nuts not to hire these guys. So, anyone up for a deathmatch? Meet me at the Malibu Club in Vice, or the construction site on Staunton Island.
It has been released before in other incarnations, it is really only good for LANs, net play is a bit laggy.