The concept of the 'do-over' is used in, at the very least, hundreds of Harry Potter stories. Here's my own take, using a HP character that actually has a good reason for getting another chance to make things better.

Harry gets whisked away into the wondrous worlds of Fairy tales. And who is his very own 'knight and shining Armor?" Why Voldemort of course. HP/LV

With Harry Potter's hearing for Underage Use of Magic looming, and Sirius Black hunted for a crime he didn't commit and was never tried for, Harry reaches out to Her Majesty the Queen for help, with unexpected results. No ships. AU starting Ch 6-9 OotP, No HBP, No DH, No Horcruxes. COMPLETE

Not enough Uke Voldie's. The pair discovers a new prophecy after Voldemort found it was a fake. Now that the tables are turned, Harry uses it to all his advantage, not to mention annoying the hell out of Dark lords. He is also introduced to the people that Voldemort fashioned himself after. Who knew? Fun ensues in a whole new adventure for Harry. Top!Harry, Bottom!Voldemort

Before the start of fifth year Dumbledore changes the plans. Unfortunately he didn't bother to inform Harry. At his trial, Harry realises that it is down to him to save his own skin. To do so his Slytherin side must come out to play, and once it's out it sticks around turning life at Hogwarts on its head. Warnings: EvilDumbles, SheepOrder/Staff, GoodGuysDontGetEverythingTheirWay

When Dean goes to sell his soul at the crossroads to bring Sam back, someone else steps in to negotiate the deal. Cas did get a sign and it sent him off into the past to change things. AU from The Man Who Would be King and All Hell Breaks Loose.

No, Dumbledore did not use a picture of Severus Snape to teach little two year old Harry to call the man 'Da.' That is a ridiculous accusation and it will not be stood for!

Harry had never been able to comprehend a sibling relationship before, but he always thought he'd be great at it. Until, as Master of Death, he's reborn one Turais Rigel Black, older brother to Sirius and Regulus. (Rebirth/time travel and Master of Death Harry)

Unforseen circumstances force Voldemort to bring Harry back to Riddle Manor with him on the night of October 31, 1981, after trying to kill him. How are our favourite Dark Lord and his followers going to deal with taking care of a baby?

What if it wasn't Dumbledore who entered the dungeon room and named himself Witness of Defence during the hearing in Harry Potter and the Order of the Phoenix?

AU. Time Travel. Snape goes back in time, holding the knowledge of what is to come if he fails. No longer holding a grudge, he seeks to shape Harry into the greatest wizard of all time, starting on the day Hagrid took Harry to Diagon Alley. No Horcruxes.

Metatron's spell to cast the angels from Heaven has unexpected consequences. Gabriel and Raphael are resurrected and Lucifer and Michael are freed from the cage. Four pissed off Archangels are joined together in one common goal, restore Heaven before the demons overrun the Earth.

Tony's a robot and always has been.

Harry has had enough of seeing his reputation shredded in the Daily Prophet and decides to do something about it. Only he decides to embrace his Slytherin side to rectify matters.

Harry has learned the prophesy and he does not believe that a schoolboy can defeat Voldemort, so he decides that if he is going to die then he is first going to live.

COMPLETE AU - Post GoF, incorporating OotP. This time, Harry, Ron, Hermione and Ginny are sent to wartorn Europe, 1943.