Gandalf

Gandalf was a name used to refer to the Doctor. When sent a message to Martha Jones via a television broadcast, he showed her the aged Tenth Doctor and sarcastically said "Say hello, Gandalf." (TV: Last of the Time Lords)

The Eleventh Doctor compared himself to the character, saying he was "the outer space Gandalf". (TV: Meanwhile in the TARDIS)

Behind the scenes
Obviously, in the real world, Gandalf is a character from The Lord of the Rings and The Hobbit by J. R. R. Tolkien. Televised Doctor Who never explicitly makes this connection.