Early versions of "Doctor Who", the 1996 TV movie, had the Doctor being the son of a Time Lord called "Ulysses" and a human woman, which would have featured significantly in the Eighth Doctor's character motivation and storylines. By the time of The TV Movie, the original plans had been changed. It is unclear if this would have been followed up on had The TV movie led to a full series.
The BBC EDAs feature references that could be referring to the Eighth Doctor being half-human.
Meanwhile "The Forgotten" (comic story) states that the Eighth Doctor used a partially faulty Chameleon Arch to fool The Master into thinking that The Doctor is half-human.
It should be noted that at no point on screen have the Ninth, Tenth or Eleventh Doctors ever been actually explicitly referred to as "pure Time Lord." Steven Moffat, in one of his columns for Doctor Who Magazine, has also stated that the Doctor is half human. Whether this means this might someday be addressed (just as he did with the regeneration limit) we do not know as yet.