In 1995, a video game was released that was unlike anything else that had ever come before it. And really, anything released since.
That game was Descent, which took the vehicle and controls of a space shooter/flight sim and dropped them into the level design of a first-person shooter.
But where most first-person shooters only exist on a plane that makes sense for humans standing on their feet, Descent's levels could branch off into all six degrees.
Imagine Ocarina's Water Temple, only you're in a ship that's underground and you're shooting at stuff. And the Water Temple was the entire game. And at the end of every level suddenly it was Return of the Jedi, as you had to flee (remembering where to go in the confusing level layouts) a detonated reactor.
It was tough, it was brutal, but it was also a hell of a lot of fun. And hey, now that it's on Steam, a whole generation of people who probably missed it can see what the fuss is about.
Especially since it's $3.50.