From Huffpo. My favorite is Apoohcalypse Now, but I love anything Apocalypse Now related.

Apoohcalypse Now:
[YOUTUBE]http://www.youtube.com/watch?v=rj_YPJvia8A&feature=player_embedded[/YOUTUBE]

Up/Gran Torino (works really well, except for the little kid's voice not matching his face):
[YOUTUBE]http://www.youtube.com/watch?v=5TuRbk-00Sw&feature=player_embedded[/YOUTUBE]

Requiem for a Toy Story:
[YOUTUBE]http://www.youtube.com/watch?v=57TiLje54hc&feature=player_embedded[/YOUTUBE]

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.