Some of these are really darn hilarious (some are old though):

Watchmen and Wall - E Mashup:

[ame="http://www.youtube.com/watch?v=B-Ka8DnUVEQ"]YouTube - Watchmen & WALL-E Mashup Trailer - 720p HD[/ame]

Shining (The Shining Romantic comedy):

[ame="http://www.youtube.com/watch?v=Z11B9L2awVA"]YouTube - shining[/ame]

West Side Story Zombie movie:

[ame="http://www.youtube.com/watch?v=x25jVzVP1bY"]YouTube - West Side Story versión Zombie[/ame]

Thundercats Mash Up "Trailer" (its fake, all the shots are from other movies... but it is damn cool in how they put it together):

[ame="http://www.youtube.com/watch?v=fb50GMmY5nk"]YouTube - Thundercats Movie trailer (fanmade)[/ame]