It's an interesting irony that although there are plenty of games based on real sports, few were invented solely in the world of gaming. The one which arguably does buck that trend is Unreal Tournament. Having burst onto the scene in 1999 as a spin-off from the original Unreal, it quickly took over as the far better game. Basing itself around a grand championship of gun-toting althletes and aliens, competing to see who was the biggest badass in the galaxy; it took the existing multiplayer modes into its own and made them brilliant.
Xbox 360