First, check out the Mozilla Popcorn tutorial:
You can click on Projects and REMIX an existing project. I modified this one in 20 seconds by changing the Google map to St.Catharines and the tweets to #Brocku:
Here is slightly more extensive remix of the Buffy Vs Edward YouTube video, looking at the bureaucratic blockades Jonthan McIntosh faced after YouTube pulled his popular video due to an unverified copyright claim by Lionsgate.
And to get really meta, here is a screencast of how I did the video with a popcorn text commentary overlay: