slowmo on 10 June 2009
Counter Strike, a mod game turned full release thanks to tools the dev provided for Half Life. Arguably the greatest online shooter and is still being played even now.
Also Half Life itself as a game that revolutionized FPS story telling. It also spawned CS, TF, Blue Shift which were great games too thanks to the good mod support.
Mario 64 - Proof that platforming and 3D worked
Sonic The Hedgehog - Single handedly built a companies reputation around the games mascot and it's "cool and speed".
