Inspired by the Zelda clone thread... what are the major games when it comes to the birth of a genre? For me, it goes in three stages... the game that first does it at all, the game that comes up with the more defined formula that actually works, and finally the game that makes it go mainstream. Two more well-known examples would be FPS & RTS.

First-Person Shooter
The First: Maze War (according to Wikipedia, anyway!)
The Formula: Wolfenstein 3D
The Definer: Doom

Real-Time Strategy
The First: Stonkers/Cytron Masters (according to Wikipedia, anyway!)
The Formula: Dune II
The Definer: Command & Conquer

So... thoughts on these or other genres? Fight it out, you know the debate is out there!