http://www.gapminder.org/ Gapminder is the site I discovered first, and like the best.
http://www.google.com/publicdata/home Google's site is clearly modeled after Gapminder. Imitation is the sincerest form of flattery.
http://www.tableausoftware.com/public/ Tableau Software hosts this site. The gallery of visualizations generated by other users is very interesting, covering every topic from tooth decay, to the growth rates of technology businesses, to the milk productivity of Wisconsin dairy cows. Moo!
http://www-958.ibm.com/software/data/cognos/manyeyes/ Many Eyes is collaborative site hosted by IBM. I need to update my Java plug-in to view these visualizations again.
When I collected these sites last year, there was another site "Swivel.com", which hoped to become the YouTube of data visualization. Unfortunately, although the content and tools were good, the business failed within a few months, after the company had spent 3 or 4 million dollars. A technology journalist commented, "These free tools on the web are easy to use, and easy to lose."
But back to Gapminder. I encourage you to explore this site, especially the featured visualizations "Health and Wealth of Nations", "200 Years that Changed the World", and "Stop Calling Them Developing Nations". http://www.gapminder.org/
Above all, these visualizations show that globally, despite everything you may read, the quality of life for people on the planet Earth is improving.
The world really is getting better.