The Apocalypse World engine was first used in Apocalypse World, published by D. Vincent Baker in 2011. The game's flexibility, as well as Vincent's open encouragement to create custom moves and content, has led to a number of hacks and games based on the Apocalypse World engine.
Three additional games are being published using this system, each by a different independent creator: Dungeon World (by Sage LaTorra and Adam Koebel), Monsterhearts (by Joe Mcdaldno), and Monster of the Week (by Mike Sands). Additional games may also be published using this system in the future.
D. Vincent Baker maintains a forum dedicated to talking about Apocalypse World and all of the hacks and designed based on the Apocalypse World engine. See the weblink below for details.