Ok so this is probably one of my favorite sites that I have found on the internet. The site has gathered information from a pile of countries over time. The information is then displayed on a graph - so you can take two different categories and have them shown. The information on the graph shown can be 'played' which will then graph the changes that occured over time. Countries can also be selected and their statistics will be highlighted. Honestly I'm having trouble describing the excellence of this site with words - so maybe an example is best.
The default graph on the site is called 'Health and Wealth' which graphs GDP and Life Expectancy of countries. As an example for a history class studying World War I you could select Great Britian, Germany, Canada and France. A teacher could then select 1900 as a starting point and 'play' the graph from that point. When you hit 1914 the Life Expectancy for Great Britian, Germany and France will suddenly drop and then begin climbing again after the end of the war. This is an excellent visual tool because it allows students to actually see the statistics that teachers often talk about.
Although this tool might be best for Social Studies teachers there are tons of different categories of statistics in areas such as environment, health, education, industry, economy etc. which means just about any teacher could find a use for this site.
Try it and let me know what you think!