As gamers continue to take their first steps into the exciting new worlds that the PS4, Wii U and Xbox One are delivering, we're still waiting on the first landmark title of this generation. The one that takes the entire industry into an entirely new direction, changing the very way games are made and played. Often, those are the games that lead the industry into a cultural design shift that affects not only developers, but players. Sometimes it's as simple as a feature being replicated or enhanced in other games, maybe even a line of clones that try to cash in on its success, but more often than not, it's the start of a revolution that sees the industry take that momentum and turn it into very real growth. It may not be the start of a new boom period, but it's certainly something that gamers always look forward to. In the past, there have been games that came out that acted as the catalyst for something new: they may have been small ripples or huge shock waves, but whatever the end result was, you could tell that the industry had just been altered in one way or another. Here's hoping that there's a game in the works that will do the same thing and send the industry into a new direction, as we'll all be there to see it. To celebrate those game changers, we've put together a list of the 20 games that changed gaming forever.