Let us think for a moment about the great masters of science. Each of these people worked hard, made the world a better place, and changed our understanding of it forever. They are well respected, totally genius, and were completely out of their minds.
Pythagoras was a Greek scholar who produced the famous equation, a^2 + b^2 = c^2. His other mathematical works considered the math side of tuning,
Oh, and he started a weird little cult that forbade many mundane things, including eating beans. Seriously. Some of the recordings about his statements on beans are somewhat odd.
Everyone knows how Isaac Newton first formulated physics into something universally applicable, co-inventing calculus in order to do it. (Newton invented calculus at the same time as another mathematician, Mr. Leibniz, both independently of each other and for basically the same reasons.) Everyone knows the famous story about how a falling apple gave him the idea to understand gravity not as a "things fall down" force as understood in the past, but to apply it to planetary orbital mechanics as well. And he was a heretical fundamentalist. Wait, what?
Yes, Mr. Newton strongly believed in Arianism, which holds God and Jesus to be separate entities. This in contrast to mainstream Christianity that believes that God and Jesus are part of the same entity, along with a third being. (Trinitarianism.) He also didn't believe in many other features of mainstream Christianity, like immortal souls, the existence of demons including a personal head demon, and Cartesian dualism. Newton had the good sense to realize that most English people at the time, including the king, were Trinitarians, and that he therefore probably shouldn't talk about religion if he didn't want his ass killed. And they would have, too, because Christianity was totally like that back then. We only know this today because he kept multiple diaries of religious philosophy, gematrias, and other kinds of whack-jobbery.
Outside of religion, Newton loved Alchemy, which was already being replaced with modern chemistry as he practiced it. (This was because almost all alchemical assumptions were wrong. Comedicly wrong.) Like most modern nerds, he had a thing for arguments, and would get into arguments over all kinds of crazy things. He's recorded as having stabbed himself in the eye once, although there seems to be disagreement as to if he was doing it to discover how the lens works, or to prove a point about control (internal using his eye muscles, or external using the needle he jabbed into his eye wtf), but in either case, not the kind of thing a normal person does.
Lastly, he is said to have claimed on his deathbead that the thing in his life that he was most proud of was that he was still a virgin after 85 years of life. Modern life would call this loserish, but it was somewhat more respected in the more religious 1700s.
A great documentary of his life was filmed in 2003, "Newton, the Dark Heretic." I especially enjoyed the acting, since they did have actors play the parts of Newton and his various associates. You can see it on youtube in six parts: 1, 2, 3, 4, 5, 6.
Cantor was a brilliant German mathematician, who proved that there is more than one kind of infinite number. For instance, all rational numbers vs. all real numbers. Both are infinite, but there are more real numbers. He also produced "Cantor's Table," the most useless possible half of a table. Also, he was terrified that someone was going to poison him and wouldn't touch food unless his wife ate some first. Wait, what?
Apparently Cantor often fell into depression, and would come out of these depressive moods feeling extra-extra-paranoid. He was quite convinced, as many paranoid people are, that his omnipresent enemies had somehow snuck poison into the food while Mrs. Cantor wasn't looking. To show how ridiculous he was being, Mrs. Cantor would take a bite of everything. When she did not die 15 minutes later, that would be proof enough and he would eat.
Unfortunately, during world war I, Mrs. Cantor died. (The Cantors were not wealthy, because being a teacher pays very very little. She probably couldn't afford the medical bills.) Mr. Cantor was at this point old (71), poor, and still totally crazy. He starved to death in an insane asylum, still refusing to eat on the grounds that the food was poisoned again.
Einstein was significantly saner than the people listed above. He reformulated physics as much as Newton, proved E=MC^2 (which was the precursor equation behind the atom bomb and all subsequent nuclear engineering), proved relativity, and has written papers suggesting that zero point energy may be possible.
Einstein was more eccentric than insane. He was said to have a closet full of the exact same copy of his suit just so that he wouldn't have to make the decision each morning. He despised quantum physics with a passion. And though I think his love of mathematical beauty is demented as a loon, the theory is beloved of almost all research scientists out there.
Tesla came from Serbia, and electricity obeyed his every whim, or so it seemed. If you ever use motors, computers, robots, logic gates, vertical takeoff planes, or polyphase anything, you owe him, big time.
When you think of mad scientists, half of what you imagine came from Tesla. He was obsessed with electricity, odd, definitely and obviously had Obsessive-Compulsive disorder. Mr. Tesla insisted on things being in 3s (numerical obsession), was terrified of things being 'dirty' (Mysophobia), and tried really hard not to touch round things directly (which is hard to do as an engineer, also wtf?).
Like Newton, Tesla was a celibate type. Unlike Newton, he claimed the celibacy was what gave him his intellectual powers. Cartoonist Kate Beaton has a hilarious take on this. (Her cartoons are often about the absurder side of historical figures. Also you should read them.)
Tesla's move to America worked out rather poorly for him. Edison ripped him off, people made fun of him, and his pet died. Many of his inventions were impossible to monetize, like radio-transmission of energy. When he died, J. Edgar Hoover promptly stole everything he ever made in the name of national security, which is only being reversed just now. (As of 2007.)
Dr. Freud is an especially interesting case. A medical doctor in his own right, he went on to develop the field of Psychology. Apparently, no one before him thought to study the human mind an how it worked. Although many of his theories were later proven wrong, his work was detailed enough of a starting point. And he was so utterly psychotic that he made the previous four people I discussed look like the very avatars of sanity, as he was the only person I can name with an Oedipus Complex, and he thought it would be a great idea to rape his cousin (although there is no evidence that he actually tried to do this outside his own skull). Wait, what?
Dr. Freud was apparently a whiny, insufferable child. His father hated him and Siggy hated his father right back. Freud got along very well with his female relatives, who were sympathetic to him. This may have messed with his unconscious, significantly. He also lived in Victorian times, which came with some really really strange ideas about women. This may have worsened his already severe psychological problems.
Like many people of his time, Dr. Freud really loved cocaine. We now know that stimulant abuse can lead to the most incredibly weird thinking on the face of the planet, but at the time people only knew that taking cocaine made them feel totally awesome.
One thing he has been proven correct about is his principle of the unconscious, that people are not genuinely aware of many of the operations that go on in their head. Many-an-argument revolves around unconscious assumptions, which can never end because the assumptions are different and unspoken.
In his honor, the phenomenon of parapraxis, in which a person makes a hilarious substitution of words, is colloquially known as the Freudian slip.