There is no most significant event in computing. What we have represents a confluence of ideas and implementations. Today, the Web is an obvious choice as Most Significant, but the Internet would not be so popular and powerful if there were not millions of personal computers available, which required the invention of the microprocessor and so on. We live in the age of the algorithm, in which enormously sophisticated and flexible artifacts are created at keyboards, yet economies and lives depend on their working. Biologists speak of analog computer modules within DNA transcription.
Physicists perform experiments with their computers first and look at the world second.
But analogy can warp the mind. When we see the world and think that controllability is the default and design always possible, then we view surprise as a bug that must be stamped out. But surprise is what makes our life journey so interesting. If there is a deity out there, we still underestimate her. As a user, I nominate Visicalc, the application that made desktop computers necessary by automating a tedious, error-prone manual task that most business people must perform and, in a tie for most significant event, the Mosaic web browser for spawning the technology that made computers an essential home appliance.