Re: The Curator.... Who is he? (SPOILERS for "The Day of the Doctor")
I think you'd have to be intentionally obtuse not to see the obvious: the Curator is the final Doctor. There's really no ambiguity. He gives Eleven(Twelve) hope that Trenzalore isn't as fixed a final resting place as it might appear.
Some say that he was the first choice to play the new Doctor but turned it down when he couldn't put racing stripes on the TARDIS.
All we know is, he's called The Stig.