Many of these have been already mentioned but for starting list I would say:
Enrico Fermi - Inventor of the nuclear reactor and one of the several leading physicists behind the Manhattan project.
Ettore Majorana - Discoverer of the neutron.
Alessandro Volta - Inventor of the chemical battery and capacitor
Gallileo Ferraris - Inventor of three phase AC induction motor
Antonio Pacinotti - Inventor of the DC electric generator
Agostino Ramelli - Inventor of the hydraulic motor
Felice Matteucci/Eugenio Barsanti - Coinventors of the Barsanti-Matteucci Internal Combustion Engine
Giambattisti della Porta - Inventor of the telescope and perfected the Camera Obscura
Giovanni Felippo Ingrassia - Discoverer of Scarlet Fever
Mauro Picone - Discoverer of the Picone Identity
Leondardo Da Vinci - Highly influential Polymath/Engineer/Artist. First to coneptualize the helicopter, parachute and inventor of the dive suit as well as a clock that kept track of phases of the moon
Cristoforo Colombo - (re)Discovered the continent of America for the European/Western world
Amerigo Vespucci - First to correctly interpret the discovery of the Americas as a novel and priorly yet unknown continent
Julius Caesar - Roman General and ruler; widely accepted as the greatest military strategist of the Western world
Scipio Africanus - Roman General widely renowned for defeating Hannibal Barca and conquering the Carthaginian trade empire
Marcus Aurelius - Emperor known for his astute rule of the Roman Empire, sound generalship and exemplary contributions to Stoic thought.
Vitruvius - Military Engineer of Julius Caesar and Architect of Augustus. First to document the creation of Roman concrete and it's unique properties of durable longevity.
Napolean Bonaparte - Military strategist, general and leader of the french revolutionary wars.
Gaius Marius - seven time Roman Consul and General known for the Marius Reforms which standardized and widely improved upon the equipment and logistics of the Roman Legions
Amadeo Avogadro - Scientist and discoverer of Avagadro's law.
Dante Alighieri - Highly influential poet, writer and philosopher. Considered the father of the modern Italian language.
Filippo Brunelleschi - Father of Renaissance architecture, inventor of the reverse gear drive and architect of Florence Cathedral.
Galileo Galilei - Widely considered the father of modern science, astronomer, early heliocentricst and created improvements upon the telescope.
Antonio Meucci - Inventor of the telephone who had created dozens of working prototypes before Graham Bell had even begun work on the idea.
Federico Faggin - Inventor of the microprocessor.
There are many more as I haven't even began to touch upon most artists and architects which are both fields Italians have historically dominated at in excellence. I also have left out the field of composers of which Venice has a particularly rich history in if I'm not mistaken. One interesting thing I find about Italy is that in nearly all time periods it remains a very strong intellectual contributor to the world in a variety of fields, regardless of how weak or strong its political dominance happens to be.
