Previously called the "QUAKE 3 Engine" and one of the most successful licensed engines in history, id Tech 3 was the engine used to create games such as Quake II Arena, Return to Castle Wolfenstein and Soldier of Fortune II: Double Helix. id Tech 3 introduced shaders, curved surfaces, 32-bit color, special effects, bleeding-edge networking and super-smooth speedy hardware rendering.
What is there to say for one of the most influential game engines out there made by the person that contributed to FPSs like no one else. Yes, I meant John Carmack. The engine itself was ahead of its time and always pushes a tear out of my eye when I see the Quake3 splash screen. I remember hours spent on this game. CoD 1 and its exp pack(DLC nowadays with tons of new stuff) was made on this engine and even though the IW/Activision dont like to admit, there is still Quake3 code in newer CoD. So you can run, but you cant hide the fact that this little engine is the father of CoD and 10 year olds talking about you mother online :)