The classic is Doctor Octopus, whose powers come from the "other" famous eight-legged creature and whose powers also involved an atomic mishap. Like Peter, Otto Octavius was also bullied and also has a gift for science. He is what Spider-Man could have easily become if he let his powers go to his head.
And Superior Spider Man takes Ock's role as this up to eleven, as he gets to literally become Spider-Man, and when confronted with the same choices as Peter, proceeds to make the opposite and more ethically dodgy decisions. Jeopardizing the public to spend more time with loved ones, winning public support by being unnecessarily brutal with criminals (even terrorizing JJJ for good publicity), forcing the symbiote to stay bonded with him despite its own wishes... it all comes back to bite him, though.
The Scorpion parallels Spider-Man's arachnid-based powers and costume. Though he is quite different from Spider-Man in personality, being a crazed villain, he still lets one guess what Spider-Man could be like if he cared for number one and didn't consider the consequences of his actions or inaction on others.
The Green Goblin parallels Spider-Man's Halloweenish or creepy style (even going so far as to throw exploding jack-o-lanterns!), has similar strength and agility, and shares Peter's sense of humor and love of adventure. If Parker had more ambition and less sense of responsibility for the well-being of other people, Osborn is what he could be. If Spider-Man allowed himself to have more fun, and care less about doing what is right or beneficial, he'd be like the Goblin.
Venom has many of Peter's powers, sense of humor, and represents what he would have become if he hadn't had the stable, loving upbringing that he had with Ben and May. Even after his transition into an Anti-Hero, Venom is still a Pragmatic Hero who more or less represents what Spider-Man could have been if he were fully willing to kill.
Ultimate Spider-Man takes Venom's role as this a step further, as they basically had the same backstory, only Eddie didn't have a loving Aunt and Uncle after his parents died to steer him on the right track or even give him a inkling of what not to do. (as with Eddie's abusive dad from 616, Carl)
During JMS's run on Spider-Man, we met Ezekiel, who had all of Spider-Man's powers but lacked the courage and determination to be a hero (he's also one for Uncle Ben in a way).
Both Spider-Man and The Lizard are both people who got their powers by accident because of science, The Lizard show just how wrong his mutation could have gone.
Mysterio reflects Spider-Man's desire for respect and his temptation to use his powers for his own selfish gains instead of helping others. Both were people who felt unappreciated and downtrodden most of their lives and started out using their special gifts for money and fame but while Peter learns with great power comes great responsibility Mysterio stays a selfish jerk.
Western Animation
Ultimate Spider-Man: Alex O'Hirn/Rhino is one to Peter Parker/Spider-Man, of all people. Both are unpopular smart kids from the same school who got mutated by Oscorp Mutagen (though Spidey got luckier); both got bullied by Flash; And Rhino even admitted that he wanted to be like Spider-Man, hence why he started drinking the mutagen. The difference is that Peter went on to use his powers as a hero and tried to help Flash be a better person, while Alex took the path of revenge and raised his hatred of Flash to creepily psychotic levels.
