Velocity Vector
OSDir has an interview with the creators of Multi Theft Auto, the multiplayer mod for the PC versions of Grand Theft Auto 3 and Grand Theft Auto: Vice City:
http://osdir.com/Article4775.phtml
They only vaguely describe their technical approach, but it looks rather interesting. It sounds like they're proxying functions in the original code in order to implement a high-level interface to the game engine. Very interesting.