Quite simply, a person who looks just like two other people.
Andy Roddick is a croppelganger of Seann William Scott and.... well no one... that's a bad example. Man, that's just as weird as Natalie Portman and Keira Knightley.
A person who looks uncannily like a mixture of two other people, so much so that they appear to be the offspring of the two other people (usually of the same gender).
"Hey, did you see that new croppleganger?"
"Yeah, she totally looks just like Kate and Jill!"
Where a doppelganger is someone's exact double, a croppelganger is two people's exact double, but fused into one person. This person also must not be related to the other two.