> anything

Perpetual motion machine to provide infinite electricity. Perhaps it could use some loophole to harvest background energy or tap into another dimension where energy is abudant. Effects: With infinite electricity machines, fossil fuels and distribution system become obsolete, all technological processes are now thousands of times cheaper and tranportation becomes free. All products/foods/tools now can be autonomously created(think space colonies), limited only by raw resources. Agriculture is replaced by decentralized vertical farms and robots. Factories switch to automated production, fueled by free electricity. Robots can run forever, all device will have permanent electricity supply. All problems with pollution/warming/etc start to disappear as energy usage switched to free electricity.

I fear that your perpetual motion machine would be promptly weaponized. I'm not sure what that would look like, but this seems to be human nature...

I would invent a time machine and go back to the night of 31 March 1837 in the home of Countess Cristina Belgiojoso to hear the two greatest pianists in history - Franz Liszt and Sigismond Thalberg - duel each other in concert.

Not a bad idea.

Transporter, as in the Star Trek series. We're talking of particles these days, but let's get it done for people and objects.

Imagine the changes to society... Instantly visiting someone on the other side of the planet, or for that matter, shipping anything to and from a colony in another planet.

An artificial womb. Would save a lot of stress for a lot of people.

An accurate history book.