Mafia, a very good GTA-a-like released all the way back in 2002, did not feature multiplayer. Now, thanks to some fans, it does.
Having been working behind the scenes for years trying to get this up and running, last week the Lost Heaven Multiplayer team finally launched a public release of their efforts, and the mod is now available for download here.
It supports up to 100 players at once, so if your drive-by tommy gun fantasies are larger than average, you're still being catered for.
Now the only problem is finding a copy of Mafia, since it isn't for sale on either Steam or GOG...(you could try Amazon but lol)