Now its been posted several times there is more the one tobi I personally believe that madara was the first and someone else the second but after browsing through some pics I saw this
http://images.wikia.com/naruto/images/1/19/Tobi%27s_left_Sharingan.png
http://images.wikia.com/naruto/images/4/4f/Madara_slightly_taking_off_his_mask.JPG

Now compare both pics together it does not look like the same person the latter is more wrinkly and looks older and the second looks younger and a different skin tone so its obvious there is a different skin tone so somewhere between itachi v.s sasuke and tobi v.s konan they have switched.
So I narrowed it down to some possible theory's which make sense.
A. The original tobi is using a jutsu similar to orchimarus and is switching bodies.
B. The pervious tobi died and his apprentice took over.
C. A clone.

Now there has been alot of debate on who tobi is obito,izuna etc...
But no one has yet provided with something sound that makes sense to my knowledge.
Now the obito theory is a pretty good one except it was tobi's left side crushed which would mean his sharigan would have been two and it seems that his right side of his body looks like it is made out of that zetsu goop not the left. Now we all gotta admit that his jutsu is very similar to kamui. So as I see it we got a couple of possibilities.
A.Someone stole obito's eye or what was left of it and regenerated it into a more usable form.
B. It is obito he survived somehow which I doubt due to the guy in the pic looking quite old.
C. It is a relative of obito since as we seen with sasuke and itachi that
their eye jutsu seem pretty similar.

I personally think its A or C I feel that the clues have been left for us but we have yet to work them out anyway I would like to hear your guys thoughts on the topic