My new favourite website is NewsDiffs.org, which tracks and archives changes made to online news articles over time. Currently it follows nytimes.com, cnn.com, politico.com, washingtonpost.com, and bbc.co.uk; no Canadian publications yet, alas. Click on the image to see how a New York Times article from today has been revised.
This website can be a wonderful resource for high school and university students in writing classes.