Ask HN: What are the biggest (simple) ideas in recent history of computing?

I'm curious which ideas you would consider the most original, creative, important, influential, etc. in recent history of IT, computer science, but to keep it still a bit focused, let's restrict examples to the web. I'm thinking especially of those ideas which don't need technical jargon to explain and are easily understood by everyone. Ideas that were under everyone's nose for a long time and in retrospect they are totally obvious if you ask the right question, but still they seemed like magic when they first appeared. To get it started, here are three that I can randomly think of: CAPTCHAs, tags, torrents.