The development of new stronger and lighter materials from which to make things. Early on, we made things from naturally occurring substances like stone, wood, and bone. Then came bronze and iron, then steel, aluminum, and now plastics and silicon.
I read somewhere that the invention that was most quickly adopted around the world was eyeglasses. They were invented in China in like the 12th century, and within 2 years had spread to India and Baghdad, and in a few more years reached Europe. Given that most geeks and the elderly wear glasses, this invention is something which is a sine qua non in the modern technological world.
Glasses spurred the development of lenses, which led to their use in telescopes and camera.
Perhaps they also changed the very way we think about disease...we no longer see health problems are a curse or something to suffer through--we just look at diseases as problems to be fixed. Because of eyeglasses there is no "stigma" attached to vision problems, pardon the pun.
Actually people drank beer (or small beer) for similar reasons long before tea was introduced to the UK. You need to boil the water to make beer, so it is generally a lot safer.
Greatest innovation was whichever of the grain crops was first domesticated enough in order for one person to be able to produce food for more than themselves - this ultimately lead to "civilisation" ie where people had roles other than food production, and gave the time for all other things to happen, as well as the possibility of more people so more thoughts could be had etc
I believe the introduction of tea to England allowed the industrial revolution to occur. Prior to that, in an ubran setting, the only safe beverages where alcoholic. Drinking boiled water made urban living safer and avoided the problems of operating the new fangled dangerous equipment while tipsy. The introduction of tea predated our understanding of water borne deseases by at least 100 years.
I would put forth Nicola Tesla and his lifetime. He was so far ahead of his time and created so many new inventions that I feel he (and his time) is the most inventive. Wireless, remote control subs, Tesla coil, Tesla Turbine, AC power distribution (over Thomas Edison's objections), the list goes on...
The fridge is a pretty powerful invention. Without it a lot of modern medicine would be near impossible and it's been one of the inventions that can be credited with having major improvements on human health by preventing food poisoning. In fact it has produce enormous change in societies that have adopted it.
Of course it still requires electricity so... maybe electricity is the greatest discovery and the electrical grid the greatest invention. Without it not much would happen these days!
It is difficult to say which are the greatest inventions, but I would like to put few here (in no particular order):
The Other Tesla David Blaza5 comments I find myself going to Kickstarter and Indiegogo on a regular basis these days because they have become real innovation marketplaces. As far as I'm concerned, this is where a lot of cool ...