Cheap, modular geothermal power using established technology from the refrigeration industry. The first commercial plants are already selling power at prices competitive with wind and half the price of solar. The technology uses water at a lower temperature than conventional geothermal so it can operate where conventional plants can't and capital costs are much lower.
The ability for people to manufacture small items in their own homes will change the entire way we look at consumer products. Design will become paramount. Big-box stores will fade away quickly, as there won't be a need for aisles and aisles of shelving devoted to the various plastic crap items that make up our daily lives.
Imagine a day when all the kids watch a show, then have the ability to instantly make their own clothing or accessories based on what they've seen. Do all the cool kids show up wearing the same thing?
Nanytes being used as a medical resource on a consumer level. Programmable to reconstruct tissue, repair frayed DNA, attack infections and cancer cell by cell, lay down neural networks for paralyzed/damaged areas the way we lay down fiber optic lines in city streets.
Also, an increase in determining quantifiable and measurable causes for mental illness - new brain scanning techniques for structural and chemistral errors, and more competant means of treating these issues than the Happy Happy Joy Joy helmet of antidepressants.
I honestly think the answer is nothing in particular, and everything in general. Single technologies do not tend to up-end the world, constellations of them change things in small ways day by day. We look at the net transformation of society over time and go "wow," but no single singularity-esque emergent event is going to change it all. Bit by bit in ways we do not see, and never expect.
I think Looneynerd is on to something. As John Aaron says in "Apollo 13", "it's all about the power".
Terrestrial energy consumption has increased beyond logarithmic scales in the 20th Century, and if civilization is going to have any chance at progressing during the 21st, it's going to have to figure out more efficient means of generating and utilizing energy resources.
So far as I'm concerned, the "game changer" will occur the second a commercial fusion reactor exceeds the "break even point" where it begins generating more energy than is required to run it. Once that happens, our entire oil-based culture becomes obsolete - not all at once mind you, since I seriously doubt fusion-powered automobiles will be immediately forthcoming - as a basis for large-scale energy production. Energy will no longer depend on who's sitting on the largest reserves of distilled bio-mass, and any country or consortium with the money and access to the technology will be in a position to exploit the new resource.
I've been thinking about memory backups. It'll start with interfacing with a computer via your brain, then incrementally replacing parts of your brain with a solid-state hard-drive, which would eventually lead to immortality, since your brain would essentially be a modular hard-drive that you can put into a new body. I think it would have to work incrementally, otherwise you run into the am-I-the-original-or-am-I-the-copy paradox.
All-purpose Kurzweil style nanites are still a long way off.
Smart nanopaerticles that have proteins on the surface that attach to particular cell types and then release drugs or toxins when triggered by light, radio waves etc. are almost here and will probably have a huge impact.
The thing we don't see coming. It will be small, easily understood by the average buyer, very handy, and hopefully only half as complex as cell phones (which I hate, and were the last Thing That Changed Everything). Youtube, for example, Changed Everything on the internet. It's simple, relatively free, and gave people a new way to communicate that they actually wanted to use.
1. Low-cost desalination and/or sewage treatment systems with self-contained, trouble-free power supplies to provide clean drinking water in the third world. 2. Europe, the US, and Canada ending the food subsidies that have caused widespread health problems at home and devastated the economies abroad. 3. Malaria vaccines. 4. Cost-effective solar power for something other than heating water in fast-food restaurants. 5. Future generations of robot scientists combined with massive databases stored in RAM and with nanotech. 6. Widespread high-quality public schools in Africa and Asia.
The thing that made me think of the Kardashev Scale was all of the engineer friends I have. Being the result of a silly liberal arts degree, I'm always asking them "Is .... possible?". The most common answer I get is "well technically yes. If we could figure out how to power it."
What a nightmare. There'd be nothing to stop someone seeding clouds with nanites designed to attach to cells and pour toxins into them. Imagine wars fought with this technology? All someone would need to do would be to airburst a bomb releasing nanites then broadcast a radio frequency and everyone who had inhaled a nanite would drop dead just like that.
Augmented Reality; wearable or implanted computer generating customizable seamless information overlays and filters, web integration, tags, remote controls.