During the Credits
There are no extras during the credits of X-Men: The Last Stand.
After the Credits (spoilers)
There is an extra scene after the credits of X-Men: The Last Stand that shows Moria MacTaggert treating a patient whose body Xavier has transferred his mind into, after being disintegrated by Phoenix also known as Jean Grey.
In Brett Ratner’s director commentary on the movie’s DVD, he explains that the patient is the identical twin of Charles Xavier, whose mind had been destroyed before birth by the power of Charles’ mutation.