13. Doom (series)
Back in ye olde tymes of first-person gaming, Doom blew people's minds. First, you had a shotgun view of the world as you ran down a hallway. Second, you had digitized death strewn everywhere. Granted, it was demons you were killing, but nonetheless nongamers were shocked while gamers were mesmerized. (Doom, by the way, was one of the first major games open for modding and released as shareware.)
Gore factor: One pint of very pixelated blood. It's easy to look at the graphics here and laugh off the bloodshed. And at the red giblets that remained after you blasted something. But remember that this was considered state-of-the-art in the early 1990s. Since that time, of course, Doom as a franchise has grown. And as the graphics of the day improved, the games rose to the occasion, becoming even bloodier than before.