Fire. That's something of a cliché answer, so here's my reasoning:<p>First of all, you probably don't interact with a bare fire on a daily basis. Most of the functions that fire used to serve have been replaced by electricity based solutions.<p>The heat of fire is a cheap and easy (compared to waiting for a lightning bolt to hit) way to cook food. Cooking of course opens a lot of possibilities in the realm of what's edible. How often do you eat raw meat?<p>Fire can be used to heat spaces, allowing people to live where it's cold.<p>Fire let's you light up dark spaces, it's a primitive sort of night vision.<p>Fire wards off predators, making it safer to sleep at night. (There was a time when large animals dangerous to humans weren't nearly extinct.)<p>Fire is a decent tool for clearing land.<p>Fire is used as part of a key step in tons of early manufacturing processes, including smoking foods, brewing alcohol, etc. (Both of those are methods of preserving foods that are cheaper than using salt. Which you can get by boiling sea water, to add another use.)<p>But really, that's just the tip of the iceberg. In addition to all of those things, Fire was (to my knowledge) the only way to generate enough heat to smelt ore, a process that should require no explanation as to its importance.
The axe: Chop trees, skin branches, kill food, skin food, kill enemies, break open shells, etc., etc., etc.<p>If you were marooned on a desert island, it would be one of the first things you'd make (a spear likely being the other).<p>Props also to the aqueduct, storage tank, settling tank, and water pipes. Those are pretty important.<p>Props to all of the above used for waste removal.<p>And vaccines. Vaccines are pretty cool.<p>Not the wheel: We had to pretty much have flattened paths (flattened by walking) or flat areas (which we eventually moved into) before wheels became useful. (There was a great article in Discover years ago about why there were no wheeled insects (if wheels are so great, why don't bugs have 'em?) - not enough flat even surfaces.)<p>Not fire, because we didn't invent it, we mastered it. So we'd have to nominate the match or the crucible or the firebrick or something.
Fire, second close is the wheel.<p>Fire gives the humans the ability to extract more energy from a given weight of food which frees time and resourses for everything else.
There is an invention without which no other invention can spread beyond its inventor. An invention which makes possible inventions beyond the ability of one person to create. An invention which permits a person to do something with their life other than kill for food, skin for clothes, and hide in natural shelters. An invention so important and fundamental, our physical brains and the thoughts of our consciousness are continually shaped by it. An invention so powerful, it defines the scope of what we can accomplish as a species. An invention impossible to leave behind or survive on most of the planet without, as defenseless physically as we are. An invention so complex and manifold, we continually reinvent it, and almost cannot teach more than the barest rudiments to any other species. Have you got it yet?
Is story-telling an invention? Because arguably other animals have languages and tools, but humans use story-telling to explain complicated things. Whether it's travel directions, technical instructions, or a spiritual message, it all gets across by some form of story-telling. And that's what makes it stick.
You may be interested in this excellent course (be warned, it's pretty controversial). To me it is more of a "documentary series" than a college course. <a href="https://www.coursera.org/course/humankind" rel="nofollow">https://www.coursera.org/course/humankind</a>
Depends of the definition of "technical" as well. If taken to mean anything that humans create, including replicating natural processes, then as other posters have said fire is the obvious choice. Now if you restrict technical to purely man-made devices it does get a bit more difficult.
in my opinion, the internet. most, if not all of humanity's knowledge at the tip of my fingers. it's with me 24/7 and almost everywhere.