How to find great restaurants in Italy
...roughly in the order I look, although it depends on the region (e.g. in Rome I lean more on local bloggers):
Slow Food Osterie d'Italia
reasonably-priced, traditional/local food
I really like their app (in Italian), which costs ~$10 and shows all the places on a map
the "Slow Food" movement is all about tradition and connection to place, so they focus on ingredients or dishes that are specialties ("typical") of a given area
I've found some wonderful places in this guidebook...but also some tired/old/impressive "institutions" resting on their laurels
it's a great guidebook for finding tradition (sometimes "tradition with a twist"), but not for people looking for innovative haute cuisine or "fusion"
if you're in Italy you can find their physical guidebook in most bookstores; in the US you can get it on Kindle (they made an English version once, in 2007, but all the rest are in Italian)
trendy / newer food
one of the top national food blogs/websites in Italy, Dissapore is great for finding newly-opened, high-quality places...unlike Osterie d'Italia, I have yet to be disappointed by Dissapore's recommendations (that being said, sometimes they are a little too "trendy")
they regularly report on "best gelaterie in Italy"
you can browse their website, but often I'll do a Google search for the region/town I'm interested in and "dissapore" (e.g. "amalfi dissapore") to see what articles come up
Scatti di Gusto
trendy / newer food
yet another top national food blog/website; they were connected to Dissapore from 2013 to 2015, and are very similar in many ways; Scatti di Gusto is probably just a bit better organized, but each site has great content
Just like Dissapore, I have yet to be disappointed by a Scatti di Gusto recommendation
...and like Dissapore, you can browse their website, but I'll often find articles via Google searches
"haute cuisine" fancy food...but also high-quality "normal" food
Michelin (yes, the same as the tire company—originally they wanted to encourage people to take car trips to restaurants) is famous for their "stars", which typically go to fancy "haute cuisine" places that are almost more art than food (often with high prices to match). 3 stars is the top award (you can learn more about the pressure to get/keep 3 stars in the documentary Three Stars [as of 2021 available on Amazon Prime]).
...that being said, their guidebook also includes a number of other, less-fancy "recommended" restaurants, including value-conscious "Bib Gourmand" restaurants where you should be able to have a starter/main/dessert for ~$40 or less
some— but not all—of the non "star" restaurants do sometimes tend to be more fancy/expensive/haute cuisine, so be sure to read the description in case that's not what you like
Michelin has a physical book that covers both restaurants and hotels (in Italian) available at most Italian bookstores, and also on Amazon.com
...but the Michelin Guide website shows most of the same content for free (including a nice Map view)...for example, here is their map of recommended restaurants in Campania
(...they also publish various guidebooks to sights, like their very thorough "green guides")
good food in general, including street food/gelato/informal food
Gambero Rosso is an Italian guidebook that rates restaurants ("Ristoranti d'Italia") but also various other food places (pastry shops, street food, olive oil, gelaterie, wine, bars...). They rate places from 1 to 3 (e.g. 3 forks = their top restaurants, which often are haute cuisine Michelin star-type places)
Ristoranti d'Italia does have an app (€8)...but it's pretty bad (and only has "ristoranti", not the other guidebooks)
...consequently I often up mostly looking at the print guidebooks
I end up using Gambero Rosso less in practice, not because I don't trust their recommendations, but because it's hard to find them (spread across multiple books and with a flaky app)...I would love it if they could create a combined app/website that showed all their recommendations
Story: I first heard about this when I was looking for a gelateria in Rome that was in an old guidebook...but found a jewelry store at that address. I asked the person behind the counter where the gelateria was, and a lady in the shop grabbed me by the hand and led me to another gelateria, along the way saying a lot of things in very fast Italian but I could make out gambero rosso. I was wondering whether she was leading me to "red shrimp"-flavored gelato, but when I got to the gelateria (Gelateria dei Gracchi, one of the best in Rome, especially at that time), I saw all the signs on the window saying "Gambero Rosso Gelaterie d'Italia" and I realized it was a guidebook! (Apparently the name comes from a tavern in Pinocchio.)
random high-quality food providers, including cheesemakers, pastry shops, and other providers of delicious foodstuffs
The name is a play on the Italian words for "glutton" and "glossary"; it's led by bow-tied self-avowed gluttons Paolo Massiobrio and Marco Gatti
They're based in Milan, so their listings are a bit North-centric, but they review places in all 20 of Italy's regions
Most of their guidebook is online but you can also find their books in Italian bookstores
Rome --> see the Restaurants in Rome page, for example Katie Parla and Elizabeth Minchilli (who also write about places outside of Rome); also websites like Puntarella Rossa
Naples/Campania --> Wikinapoli! ...but also:
Luciano Pignataro - based in Salerno so a lot of his recommendations are in Campania, but he also writes about other areas around Italy
Foodding - nice blog, mostly about food, mostly in the Naples/Campania area, with interesting restaurant recommendations
Karen Phillips' AndiamoTrips - Karen runs food & wine events, but also blogs about restaurants (largely in the Naples area)
Kristin Mella's Napuleats on Napoli Unplugged - Kristen has a bunch of posts about restaurants in the Naples area
Mangia Napoli Flood Blogger - Facebook feed with some pretty good stuff
Touring Club Italiano (TCI) - this guidebook (both online and offline books) recommends a host of things (not just restaurants), but I've seen them recommend so many mediocre restaurants that I just don't trust their judgement without a second opinion
Lonely Planet/Frommers/Fodor's/Rough Guide - each of these guidebooks can be helpful but not always: their quality really depends on who wrote/researched the latest revision, and how recently it was actually revised (versus republishing the previous year's research)...I've rarely found great restaurants from these guidebooks, but sometimes they find some gems (although they can be useful for other stuff...Rick Steves does a nice breakdown of the pros and cons of each publisher here.)
Moon - I've found these guidebooks to be super-helpful (e.g. the Amalfi Coast one is written by Laura Thayer who lives here); that being said, for restaurant research, the information can be stale (since editions only come out every few years)
Time Out - this used to be my favorite guidebook for food in Italy, but it seems to be getting out of date; their online listings are generally worse than the books
Rick Steves - I love love love Rick Steves' books for showing you the highlights of a given town or museum...but their food recommendations tend to be a bit tired/out-of-date.
DK Eyewitness Guides - perhaps the most useless guidebooks (apart from general inspiration); elsewhere on the site I talk about why they're overrated
Identità Golose - not a bad guide (their English version is often a ~year behind the Italian one) from an group that also organizes events with chefs, but their guidebook seems to have a lot of overlap with Michelin so it's usually not among my first references
How to Use These
Especially in touristy areas, restaurants have learned how to game the system, so don't make a choice simply because a particular restaurant is "#1 on TripAdvisor"
...that being said, if the average score is high or low, that's probably a good signal of how much people like the place
I've noticed that inexpensive to-go places (like sandwich and pizza shops) tend to be popular on these websites (perhaps because they're cheap & easy?)
Note that there might be a big difference between local vs. tourist or Italian vs. non-Italian reviewers...for example, I've seen amazing restaurants bashed by American tourists on TripAdvisor for things like "The pizza didn't have enough cheese on it, the soda didn't have ice, and the waiter didn't bring our bill until we asked for it"...all normal things for Italian restaurants.
I often find new places that aren't in the guidebooks just by scrolling around on the map—you can also do Map searches for specific dishes or cuisines (e.g. "japanese restaurant")
Google makes it relatively easy to filter by average review score; typically I avoid places with lower than 4 out of 5 (since even with the caveats above, a good place should be able to maintain at least that)
I really like the "Photos" feature, since it's a quick way of seeing what the food & ambiance is like: Is it humble "homestyle" (casalinga) pasta in a bowl or fancy haute cuisine spheres on giant plates? Do their dishes emphasize vegetables or meats? Does the atmosphere seem kid-friendly or not?
They let you see the "rankings" of the restaurants, but keep in mind the caveats above...and that they have some special algorithm that makes places with lower averages sometimes rank higher.
As with Google, I take the rankings/ratings with a grain of salt and mostly just avoid places with very low rankings
You can also filter by language, which can be useful to see what Italians vs. tourists think of a place
Yelp is not very popular in Italy, but sometimes has good information (mostly from Americans reviewing places)
Il Mangione is a local Yelp equivalent but I haven't found it all that useful
Instagram can be an interesting way to find interesting places to eat (and visit), if you search by tagged location (works better for smaller towns/areas)