The year is 1998. A previously unknown group of game developers known as Valve Software has just released Half-Life. Of course it becomes an instant hit, entertaining a generation of gamers worldwide. But beyond that, buried away on the CD-ROM inside that ridiculously big cardboard game box is a program called “Worldcraft”. According to a note at the back of the instruction manual (remember those?) this program would allow fans to “create a replica of their garage or a fantastic alien world”. And create they did. With this tool, now known as Hammer, at their disposal, the possibilities have been almost endless. From Counter-Strike to Ricochet, from Black Mesa to deep space to World War II France, the community created a tidal wave of new adventures, far beyond what anyone might have anticipated. Fifteen years later, we show no sign of stopping.