The Arma II-mod DayZ is one heck of a gaming success story. Bohemia's in-development standalone version of the game has almost sold three million copies thus far, and it's only getting more and more popular as time goes on.
The developer did this with an marketing budget of exactly zero money. Producer Brian Hicks talked about this phenomenon with PCGamesN yesterday: "My personal opinion, since we have nearly $0 marketing budget, our marketing is me and Dean going to shows and interacting with consumers, I'd say Youtube and Twitch are the entire force behind DayZ's success."
The game, while still just a mod for the military shooter, came to prominence via let's plays and the like, and through no small amount of word-of-mouth. The success of the mod inspired Bohemia to hire its creator, Dean "Rocket" Hall, to head up a team working on a standalone version of the game, which as mentioned before is currently selling well on Steam Early Access. It is expected to be finished sometime in 2016.