"Most interesting" seems like a pretty subjective metric. The advances that come to my mind are things like agriculture, calculus, the scientific method, Newton's theories, representative democracy, post-modernism... those all resulted from decades of effort by groups of people who focused on that task specifically.
Tools are super valuable; it's arguable that many of the things I list above are sort of tools. I just don't think that a tool that is quickly slapped together in the service of another project is likely to be a game-changer.
In fact, I'd say that one of the keys to growing your personal impact is to find the best work that's already been done, and use it to further your goal. Even Newton said he was standing on the shoulders of giants.
Calculus of all things was not an end by itself, but a set of secret tricks by Newton to prove results about the orbits of planets (that then he translated into a more cumbersome geometrical language)
Then there are the three branches of modern mathematics, according to V.I. Arnold:
1) modern hydrodynamics and PDE, are just guys procrastinating while they are paid by the manufacturers of nuclear submarines
2) celestial mechanics, control theory and ODE, are just guys procrastinating while they are paid by the manufacturers of rockets and missiles
3) number theory, are just guys procrastinating while they are paid by the CIA to develop cryptographic tools
agriculture : this kid is just playing with small seeds instead of collecting nuts for winter
computers : just tools to speed-up the important computations of nuclear reactors
TeX : the guy just had the write a damn book about algorithms but spent 10 years procrastinating
Bach : the guy just wanted to glorify God
I stand by my point that nearly all important advances of mankind are tools developed to solve a long forgotten goal.
The idea that calculus was some sort of side project for Newton both diminishes the level of focused effort he put into it, and ignores the contributions of Leibniz and the many other mathematicians whose work was known and incorporated by Newton and Leibniz.
If we just open up the idea of "tool" wide enough, everything becomes a tool. Writing a book isn't actually a goal, it's just a tool for acquiring money and fame. Calculating the orbit of planets was just a tool for Newton to establish social credit and historical significance. Agriculture is just a tool for eating, but eating is just a tool for living, but living is just a tool for procreating, but procreating is just a tool for perpetuating genes.
Why are we perpetuating genes? Really, no one knows. Solve that problem, I guess, and finally we'll have the one project enriquto can't easily dismiss as tool development.
"I stand by my point that nearly all important advances of mankind are tools developed to solve a long forgotten goal."
That is indeed true.
Of course, almost all of the goals I have accomplished for myself have been accomplished by actively engaging in pursuing that goal rather than pursuing some side channel, while the side projects I've created have generally been a lot less useful to myself and humanity than, say Bach. :D
Tools are super valuable; it's arguable that many of the things I list above are sort of tools. I just don't think that a tool that is quickly slapped together in the service of another project is likely to be a game-changer.
In fact, I'd say that one of the keys to growing your personal impact is to find the best work that's already been done, and use it to further your goal. Even Newton said he was standing on the shoulders of giants.