Must-Have Apps in Italy?

CityMapper

Whatsapp. That is how all of my hosts and drivers wanted to communicate.

Uber worked well in Rome and Florence. It did not work well in Naples.

We found the EasyPark app very useful --- you can even change your plans and remove or add minutes.

Che Bateo for Vaporettos in Venice

Yeah, if you are staying in lodging other than a hotel, then Whatsapp is important, allows you to communicate with each other without international charges.

I'm just as happy with Google Maps as opposed to Citymapper, but if you have an iPhone, it is better than Apple Maps.

If you are taking trains, the Trenitalia App is handy to buy regional train tickets and any others you did not buy before your trip

If you are heading to any museums or sights, look online to see if they have an app. For example, the Pompeii site has "My Pompeii" which serves as an audio guide, it will save you time and data to down load now as opposed to at the site. There is one for the Uffizi as well, and becoming more and more common at lots of places.

Public transit everywhere:
Moovit

Trains:
Trenitalia app and Italo treno app (Italo Treno operates only high speed trains in main lines)

Taxi Rome:
ItTaxi (owned by Italian RadioTaxi association)
Uber (has an agreement with ItTaxi so you can use it to summon taxicabs, but they add 3€ per ride)
FreeNow (same as Uber, they add a commission)
AppTaxi
TaxiMove (for Florence, however you won't need a taxi in Florence, which has a small city center, except for from/to train station if you have luggage)

Thanks for sharing!

I would add that Rome2Rio is helpful as well and All Trails if you plan to do any hiking. Google Lens also good for identifying a multitude of things. Just aim your camera at what you want to know more about and Google will tell you all about it. And how could we forget the Rick Steves Audio Europe app. Has all kinds of guided walks of major sites and neighborhood walks and so much more. Many of these guided tours could actually eliminate the need for booking a guided tour. Just remember to bring your earbuds so that you can listen through your phone. Also a good idea to download the guided tours that you're interested in before you leave home so you have them on your phone in case of spotty internet.

Toilet Finder for Android and Flush for iPhone are pretty self-explanatory and may be helpful, especially in Italy.

Viator is great for booking tours, either before you get over there or on a whim.
And the last one is WanderLog, which is an excellent app for organizing your complete itinerary before you leave home. Very easy to organize hotel, air, tours, sites you want to see.

I much prefer Trenit! to the official Trenitalia app. Much quicker response to schedule changes, and at least for me a friendlier user interface. And it contains the same information, including Circumvesuviana and other regional service. But it's simply a preference thing.

I'll add that Google Lens works remarkably well in combination with Translate when observing posted signs in unfamiliar languages (as Italian is for me). It's almost like magic. It was especially valuable for us in Poland.