Rockstar Advanced Game Engine
From WikiGTA - The Complete Grand Theft Auto Walkthrough
The Rockstar Advanced Game Engine (RAGE) is a new engine, which is made by Rockstar Games itself. It is already used in the game Table Tennis. GTA IV will also run on the RAGE engine. Rockstar anounced that it will use the RAGE engine also in several upcoming games, such as the racing game Midnight Club: Los Angelos and the detective game L.A. Noire.