Galileo Galilei, the father of science, the scientific method, physics, modern physics, and astronomy, among other things (including 2 daughters and 1 son, all of them out of wedlock), was born on February 15th (Julian calender i.e. Old Style), 1564 in Pisa, a town in the Duchy of Florence.
His father Vincenzo Galilei was an achieved lutenist (like an obsolete guitar player) and music theorist and the family was doing fine. Nevertheless, this father needed lots of money for some dowries and extra expenses needed for Galileo's younger brother Michelagnolo Galilei (another lutenist, one who never earn any real bucks with his music). Despite the relative wealth and fame, one could say that Galileo (who became a lutenist himself) needed extra income and many of his early inventions were actually motivated by the thirst for extra money.
Despite his prestigious background, I would count him as a self-made man who shared many of the typical character features with great folks who come from poor families.
When he was in his late 20s and early 30s, he would study why the pendulum had a constant period and how it depended on the length/geometry. He created a thermoscope (a father of the thermometer), studied hydrostatic balance, and was hired as the boss of the maths department in Pisa, before he moved to Padua to teach mechanics, geometry, and astronomy. He would study the acceleration by freely falling objects from the tower of Pisa. Does the velocity increase by the same amount per unit time or unit length? He was able to settle this question in the scientific way. Yes, it's per unit time.
He wrote not only Dialogue Concerning the Two Chief World Systems but also other masterpieces of the polemical literature. At some moment, he would become a staunch heliocentrist who wasn't afraid to point out that the Pope was an idiot. Some people tried to resist this insight; see Galileo as the tragic hero for comments about the trial as well as Galileo vs Benedict XVI.
Dozens of other blog entries about Galileo have been written, too.
Some of his testing questions that helped him to understand early physics better were concerned with the tides (it was in the pre-Newtonian era so there were still some major technical mistakes in his answers: for example, he couldn't understand why the tides have two periods per day and not just one); the 1604 Kepler's supernova (he could show that it was a distant star – no parallax – which contradicted some bizarre Aristotelian dogmas about the immutability of the heaven – the heaven is defined as the things that are "infinitely far" and those should never explode); the Milky Way (he appreciated it's there and what it is probably made of); and observations of other planets and their moons, some of which he observed for the first time with his telescope that was one of the first good ones. He was not necessarily the guy who invented telescopes with all of their know-how but much like great IT industrialists today, he knew how to put these ideas floating around to work.
Aside from the telescope, he invented a geometrical and military compass and lots of various gadgets for sea navigation etc.
There's way too much to say about Galileo Galilei so I gave up any plans to be comprehensive at the very beginning. But I want to say the following thing. Galileo Galilei wasn't infallible and his comments about particular questions in physics and astronomy weren't always 100% right. However, he was right about the key things and more importantly, he pioneered the very scientific method that allows us to be increasingly right and accurate. In a sense, the scientific method mimics Galileo's own life. It isn't producing flawless results from the very beginning; it is a process to gradually eliminate the flawed hypotheses and – with the help of observations and experiments – converge closer to the correct ones. This methodology to learn about the truth was revolutionary and, to a large extent, new – even though it looks obvious to us today. By finding this method, he established a new religion of a sort, one that actually works in its goal to make progress.
The reason why he couldn't get as far as Isaac Newton almost a century later was that unlike Newton, Galileo was no ingenious mathematician. His knowledge of maths was "standard" at most and he didn't make any major breakthroughs in maths. And this does slow down everyone who would like to become a revolutionary in physics. In maths, Galileo became famous for childish insights of recreational mathematics such as the observation that the set of perfect squares has the same number of elements as the set of integers, despite their being subsets of one another ("Galileo's paradox"). That's too little relatively to, say, Calculus.
Although Galileo's contributions had to look formidable even during his lifetime, he was often harassed for having contradicted some dogmas, stereotypes, and the authority of the lesser minds. He was tried as a possible heretic and the verdict was a life in (home) prison. He was arguably lucky not to be executed. Centuries later, the Catholic Church and others were gradually rehabilitating him.
While Galileo played a key role in the birth of science (or modern science) or physics as we know it and even though he was clearly versatile and extremely skillful and smart in many different ways, I would say that the bulk of these achievements depended on his unusual courage, self-confidence, political influence, and his ability to provoke rather than his being one of the smartest men of all time. In particular, the very fact that he could get away with this self-evident heresy (according to the traditions at that time) was very important for the Western civilization that could suddenly notice that some important ideas may arise from corners that aren't fully endorsed by the church's group think. This "relative victory of a heretic" brought lots of meritocracy – desire to find independent-of-religion, new methods to judge the validity of claims – into the smart folks' thinking. Don't get me wrong: he was extremely intelligent. But the intelligence was nowhere near Newton's. But that doesn't matter for us; he has still done amazing, unprecedented things and it was up to him how to achieve those things.