Feels like every manga forums needs a discussion about this topic :3
Well there's been LOTS of discussion about Obito Uchiha, old friend of Kakashi, being the same person as Tobi, and here's what I've seen:
Obito is Tobi:
1. Tobi is just Obito if you take the name apart, O-Bi-To, To-Bi.
2. Tobi has one side of his face covered.
3. Tobi and Kakashi has pretty much the same Mangekyo Sharingan ability, they both have something to do with bending the space and teleporting things somewhere else.
4. Obito's right eye was given to Kakashi, there is a possibility that he took someone else's eye, and hidden it until he absolutely had to use it for the Izanagi like what Danzo did.
5. Since the Mangekyo Sharingans have pretty much the same ability, there's a possibilty that Obito's face wasn't completely smashed.
6. Tobi has the same haircut as Obito.
Obito is not Tobi:
1. Kakashi was still a teenager when the Kyuubi attacked the village, and Tobi at the time was a full grown adult, big age difference.
2. The rock smashed right side of Obito's face.
3. Obito was later crushed by boulders.
4. Obito had just awakened his sharingan right before he died, and Tobi had the power to control the Kyuubi.
Feel free to add on to what I've already put here