Half-Life celebrates its 15th birthday today. Valve's genre-exploding, literal game-changer first appeared on the 19th November 1998, taking the well-loved first-person shooter and crafting something extraordinary. It was considered a turning point. A new bar for games to beat. And one that was safely broken by, er, Half-Life 2 six years later. Below are some of RPS's favourite memories of the old, old game.