If you intend to read Harry Potter and the Half Blood Prince (HBP), and you don't want to know how it ends, stop reading right now.
I have a theory that explains the central mystery of the Harry Potter books: what saved him when Voldemort tried to kill him as an infant, after murdering his parents.
In the HBP, we learn that Voldemort was trying to achieve immortality by splitting his soul into seven parts. A horcrux is a repository of a fragment of a soul. Creating a horcrux requires an act of supreme evil: a murder. The puzzle at the end of the HBP is what are Voldemort's six horcruxes.
Here it is: Harry himself is a horcrux.
The known or suspected horcruxes are (HPB ch. 23):
1. Tom Riddle's diary, which Harry destroyed in the Chamber of Secrets.
2. Marvolo Gaunt's ringe, which Dumbledore destroyed as a horcrux, blackening his hand in the process.
3. Slytherin's locket, possibly destroyed by the mysterious R.A.B..
4. (suspected) Hufflepuff's cup.
5. (suspected) An unknown object of Ravenclaw's.
6. (suspected) An unknown object of Gryffindor's.
7. (suspected) Nagini the giant snake.
We know that Harry's mother Lily was very gifted at charms. We also know that after Voldemort killed Harry's father James, he intended only to kill Harry, and he only killed Lily because she refused to stand aside. Voldemort's intention when trying to kill Harry was to fulfill the prophecy that only one could live of Voldemort and a wizard born at the end of July.
So here is my theory: Lily and James Potter were on to the horcrux plot. We know from the note in the fake locket recovered by Harry and Dumbledore that the horcruxes were known to others besides Voldemort. I think Lily created a charm that caused Voldemort to unwillingly and unknowingly create a horcrux when he killed her, and that Harry was the horcrux thus created. So when Voldemort tried to kill Harry, he was unwittingly destroying his own horcrux. This caused the spell to backfire on himself, and destroy his physical body.
This would plausably explain many things such as Harry's ability to speak Parseltongue, and Harry's ability to sense Voldemort's thoughts and feelings.
It also creates a terrifying conflict for the final book: In order to kill Voldemort, all his horcruxes must be destroyed first. How does Harry do that if he himself is a horcrux?
By the way, I think the theory that Dumbledore is not really dead (see
http://www.dumbledoreisnotdead.com/introduction.html) is a load of hooey. Mostly because the last book would be more dramatic if Dumbledore is dead. But it's a creative theory, though. Maybe Dumbledore and Snape both took polyjuice potion to become each other, and Snape is dead. That would resolve the issues with the Unbreakable Vow he made with Narcissa.
Also by the way, I think R.A.B. is Regulus Black.