The Amazon Kindle, Kindle for iPhone and Kindle for iPad each provide a very simple mechanism for adding highlights. Every month, Kindle customers highlight millions of book passages that are meaningful to them.
Amazon combines the highlights of all Kindle customers and identifies the passages with the most highlights. The resulting Popular Highlights help readers to focus on passages that are meaningful to the greatest number of people. They show only passages where the highlights of at least three distinct customers overlap, and do not show which customers made those highlights.
There is also a section identifying the most heavily highlighted passages and books of all time.
You can peruse the results at this link.