Most of what's been suggested so far are expendable technologies. Bits and pieces based on other technologies that tomorrow, might be obsolete or replaced by something better. Where a real world, wonder of the world is Niagara Falls or the Grand Canyon - ie: something that has been here roughly forever and will be for some more time (hundreds of years)
I'd say:
First and foremost - the modern day processor. Moore's Law has yet to fail - stronger and faster processors continue to come out every year and the technology progresses as a reasonable pace. Bigger and better processors allow us to write ever more complex code to run on them.
2nd - The internet. With no single point of failure, it spans the entire world, allows an array of communication services and entertainment from sharing medical research to pictures of cats. It would be incredibly hard to replace it and the world as we know it would be quite different if it went away.
3rd - Email. While I argued above that specific technologies can be replaced, email has been around from almost the beginning and will continue to be. Use Slack, FB Messenger, whats app, etc... but email is still the #1 means of communications for 99% of companies. It hasn't changed much; probably won't change much still. Many have tried to revamp it (IBM to name 1) and failed. It can be fickle - trying to write html that looks great across hundreds of email clients, but it's still a reliable and basic means of communications.
4th - cloud hosting / cloud computing. A very new concept and technology but unless your a bank, major brick and mortar outlet or research facility, gone are the days of calling Dell or HP, ordering a dozen servers, provisioning them, maintaining them, etc... Cloud computing has allowed the smallest of startups to scale up and scale up fast. For 99% of the world, cloud computing is the way to go for the foreseeable future.
5th, 6th, 7th?