Who is Tobi?

Kishimoto is Tobi.  Imagine that, Tobi removes his mask and then poof!  An actual color picture cut -out and glued on to Tobi's face.

But if he wanted a more serious role, it would have to be Madara's brother.  First of all, there is no proof that he actually died.  We see his eyes removed and a coffin with the Uchiha crest on it.  That is all.  Second, Tobi doesn't seem to care about losing and replacing eyes.  This is not normal so I would think that if he had already lost his eyes to Madara, he wouldn't care about using other people's eyes.  Third is obvious, Tobi's knowledge of the Uchiha and the entire ninja world.  Being extremely old and from such a skilled clan, Madara's brother would have oddles of knowledge about the shinobi world.

Fourth, Tobi helped Itachi kill the Uchiha clan because he held a grudge against the Uchiha.  This grudge probably stemmed from the clan's abandonment of Madara, who's death could be related to the Uchiha and Madara going their seperate ways.  Fifth, Tobi pretending to be Madara was probably just a way for people to reconize him without having to explain that he is actually Madara's brother.  It's like living in a small town right outsie a major city and just saying your from that city to simplify things.  He figured that they were similar enough that impersionating him wouldn't make much a difference and it would also inspire fear.Would you be more impressed to meet President Obama or Obama's brother?  It's the same kind of deal.

Sixth, Madara's brother could have known Madara was going to fight Harishima and went to the battle scene afterwards to recover both Madara's body and some Harishima DNA.  In fact the rinnegan eyes given to Nagato could actually have been Madara's!  Seventh and most powerful, Madara and Tobi seem to be related.  Nothing freaked Tobi out more than seeing Madara resurected.  When Madara was resurected, he seemed to know what Tobi was doing as soon as he heard the kages mention "the other Madara."  There is no way Madara would know what that meant unless they knew each other and were probably working together at some point.

Let me acknowledge some other theories and prove them wrong.

Tobi is a Zetsu clone:  Really? no if anything it's the other way around.  Zetsu could be a manifestation of Harishima's DNA mixed with some crazy jutsu by Tobi.  But Tobi bosses Zetsu around so much that I think Zetsu's only purpose is to serve Tobi.  That isn't much of a life, unless of course Tobi made Zetsu and therefore owned him. 

Tobi is Shisiu (Itachi's friend):  I thought this was true too because it was mentioned by an Uchiha that Shisiu was the best at teleportation jutsu (Tobi's trademark).  But recently, Itachi's flashback showed Shisiu dying to protect the Uchiha and the leaf village.  Giving Itachi his eye as well, Tobi would never do that.  Plus the conversation Itachi has with Tobi is totally different from the one with Shisiu and suggests they are different people.

Tobi is Obito: No, just no.  What is the motivation?  How could someone old enough to harvest Harishima's DNA appear 12 at the time when Kakashi is around 12.  He is about 26 now so that's only a fourteen year difference.  26 is pretty young to be around when Madara was relevant.  Plus,, the way he died, giving Kakashi his eye, Tobi would never do that.  The only real evedence to support this rediculus claim is that Obito is an Uchiha, the names are similar, and when Kakashi was about to use his Mankekeyou on Tobi, Tobi said, "don't bother Kakashi, that jutsu won't work on me."  This implies he is knowledgeable about Kakashi's eye powers but he could have found that out anywhere. Kakashi has used it against the Akatsuki at least three times so far (twice on Diedara, and once on Pain).

So now you know.  Madara's brother is running around trying to attain the peace his brother could not acheive.  In fact, Madara running form the village saying the Leaf and Uchiha could not get along is very similar to Tobi saying true peace could never happen.  So unless Tobi is Kishimoto, you get bet it's Madara's brother.