I would imagine that the use of chert to make cutting edges from flint, probably made a major impact on early human lives, and the species' subsequent development. It was also used until at least the 16th century as a building material in some places.

It is possible, by hand, to make a few feet of string from wool, if you are patient about rolling it against your leg (like you'd roll dough out into a snake from a ball). The spindle--a stick with a whorl (weight) attached, preferably by a hole through the weight--means you have somewhere to wind up yarn as it is produced, so you can produce lengths longer than a few feet.

This would have been many thousands of years ago, of course.

The first thing that came to mind was the abacus, yet I don't know if that's as simple as you were wanting. But I'm sure that really changed everything, especially architecture.

While not a hand tool, the plow is pretty simple, and revolutionized agriculture.