Gandalf

Gandalf was a nickname the Doctor sometimes bore. When the Master sent a message to Martha Jones via a television broadcast, he shown her the rapidly aged Tenth Doctor, jokingly saying "Say hello, Gandalf." Later, the Eleventh Doctor jokingly called himself "the outer space Gandalf". (DW: Meanwhile in the TARDIS)

Behind the scenes
Obviously, in the real world, Gandalf is a character from The Lord of the Rings. However, televised Doctor Who never makes this connection, and it's unclear whether the broader DWU does, either.