Here's one I've been working on, which I'm proud to say managed to take out top honors in a contest over at CGChannel. The competition was run by Jonathan Berube, who kindly provided a great plate to work from. The task was to remove any signs of the modern world, then add Rivendell and modify the environment until it looked like it sat in the world of the Hobbit / LOTR films.
You can see the original plate on Jonathan's Flickr page if you like, so you can see where my work began and ended.
I've still got some work to do on this one though as it's not quite finished. I'll be sure to post a link to the video once its all done!