The Curator

The curator of the National Gallery in London was a mysterious individual who had insight into a particular Time Lord painting. He spoke with the Eleventh Doctor and told him that the name of the picture was in fact Gallifrey Falls No More, and pointed the Doctor in the direction of a search for Gallifrey.

The man then made some ambiguous references to possibly being the Doctor once, or that the Eleventh Doctor was just a version of him. (TV: The Day of the Doctor)

Behind the scenes
The script never directly states that this character is an incarnation of the Doctor. But it does certainly allow that possibility. Further fuelling the possibility is that Tom Baker is not separately credited for this performance, receiving only a credit for playing "The Doctor".