From Huffpo. My favorite is Apoohcalypse Now, but I love anything Apocalypse Now related.
Up/Gran Torino (works really well, except for the little kid's voice not matching his face):
Requiem for a Toy Story:
There's a bunch more at the link.
The 13 Best Movie Mash-Ups The Internet Has To Offer (VIDEO)
I think they're really well done and obviously a lot of thought and time went into them.