If there's one thing everyone knows about Spider-Man, it's that he covets his secret identity more than just about any superhero around. He does this to ensure the safety of the people he loves, which suggests he would never let anyone know who he really is underneath the mask. Despite this, his secret identity has been one of the worst kept secrets in comics since Clark Kent bent over and lost his glasses down a sewer grate.
Over the years, Peter Parker has revealed his secret identity to tons of people, be they allies or villains. Sometimes, his secret gets out when he's not aware, while other times, he gallantly pulls his mask off to reveal who he is to anyone standing around. The movies have handled this a bit differently than the source material, so if you aren't as up to date on the comic books, you may not know of all these times Peter Parker's biggest secret has been revealed.
Peter Parker's biggest secret was first revealed to the world all the way back in The Amazing Spider-Man #12 by none other than Doctor Otto Octavius himself. The fight kicked off when Doc Ock took Betty Brant from the Daily Bugle office. He told Jameson to print a challenge to Spider-Man in a vain attempt to fight the superhero to prove his superiority. At the time, Peter was suffering from some sort of viral infection, which caused some hiccups to his superpowers. This led to Doc Ock overpowering Spider-Man in a surprisingly easy and short fight which climaxed with the image above - Spider-Man's mask was pulled off to reveal none other than Peter Parker.
Since the fight was so easy, and nobody could believe Peter Parker was actually Spider-Man, everyone present pretty much dismisses the revelation with the explanation that the "real" Spider-Man ignored the challenge, which pushed Pete to stand-in so he might be able to save Betty. Peter then goes to bed and wakes up feeling refreshed and superpowered again, which leads him to fight his nemesis once more. Everyone kind of sets aside what Pete did, and his secret remains hidden - for a little while longer at least.
Spider-Man has fought some of the biggest bads in the Marvel Universe, but one the one that constantly finds a way to bring him down is a simple head cold. Whenever Pete got sick in the early days, he would either lose a superpower or just be about as useless as any normal guy with a head cold. In The Amazing Spider-Man #39, another cold managed to deaden his senses enough that his Spidey-Sense failed to warn him that his arch-nemesis, the Green Goblin, was watching him change out of his superhero garb, and into his civvies in an alley. Granted, it wasn't just the cold; a new kind of gas Goblin used against him helped to deaden his senses further, and coupled with the cold, Pete was completely unaware he was being observed.
This is how Gobby learns the secret of Spider-Man's true identity, and a short time later, he confronts Spider-Man by calling out the one thing he hoped never to hear while wearing his costume, "Peter Parker." Pete leaps onto his enemy and tries to take him out without the use of his web-shooters but manages only to bet suffocated and knocked out. This leads to Pete being tied up and carried via glider through the city until he is taken to Goblin's lair. At the end of the book, the biggest secret of all is revealed when the Goblin unmasks to reveal he is none other than Norman Osborn.
This issue would have disastrous consequences for years, and the development of Goblin as Spider-Man's greatest foe would stem from this point. He would go on to torment the web-slinger, and even go so far as to slay his girlfriend almost 100 issues later. Over the years, Norman's particular brand of evil would keep the secret buried, but every so often, it would come out again to wreak havoc upon Peter's life. This revelation was truly one of the big ones.
In The Amazing Spider-Man #87, Pete is once again suffering from the flu, and it's a bad one because he loses his powers once and for all! At least, that's what he thinks when they completely fail him. He's on his way to Gwen's birthday party but realizes he has no gift for her so he does what any boyfriend would, and breaks into a jewelry store to steal some pearls. Realizing how idiotic such an act is, he returns the pearls but manages to stumble out of the window to fall on the ground, powerless.
This leads him to walk into the party, which had ended some time ago, leaving only Harry, Gwen, her father, and Mary Jane to find Peter walk in holding his Spider-Man mask. He tells them he is/was Spider-Man, but his career is over! He then runs out and heads to the hospital where he checks himself in as Spider-Man. The thinking is that no other person would have radioactive blood, and he was concerned the radiation was finally hurting him. They end up giving him a clean bill of health, and as it turns out, he's fine and has gotten over the cold - but what to do about his friends?
He ends up enlisting the aid of Hobie Brown, otherwise known as the Prowler, to put on his costume and pretend to be Spider-Man so he can "prove" he wasn't truly the web-slinger. He ends up explaining away his earlier revelation as a result of his flu-borne delusion. His buddies are skeptical until Hobie shows up in costume to back up Pete's story, and everything goes back to normal - for a few issues anyway.
Throughout his superhero career, Pete has suffered some significant losses, which have had a great impact on his life. The first were his parents, but the most important is his Uncle Ben, which started his life as the Friendly Neighborhood Spider-Man. The third passing to really impact who Peter Parker is as both a man and a superhero is the loss of Captain George Stacy, his girlfriend Gwen's father.
The end comes about in The Amazing Spider-Man #90 thanks to longtime Spider-Man rogue Doc Ock. The two fight in the beginning of the issue, which leaves Pete beaten and exhausted. He ends up passing out at Gwen's place where he wakes up to Gwen and George. At this point, it's apparent George may know his superhero secret as he comments on how quickly he recovers, but seeing as he doesn't mention anything and hasn't exposed him, Pete shrugs his suspicions off for a while.
Later, Spidey and Doc Ock fight once more upon a rooftop where Spider-Man manages to web his foe's tentacles. This causes them to flail violently, and they end up smashing a nearby chimney, which sends a massive pile of rubble to the ground. Captain Stacy sees a child about to be hit by the rubble and throws him out of the way but winds up taking the hit instead. Spidey rushes down to save him but is too late. He scoops up the man and takes him to safety, but as he is about to pass, George reveals he's known Spidey's secret all along when he asks him to look after his daughter. This event leaves Peter feeling as much guilt as he did over the passing of Uncle Ben since George has become a new father-figure to the young man. It's a devastating blow, which will stay with him for the rest of his days.