Via FlowingData, a great visualization by Ben Fry.  Fry looks at how The Origin of Species changed from the first edition to the sixth edition, noting which passages were added, removed, edited, etc.  You can watch the text change by edition as well as scroll over each section to see the actual text by edition.

I’d love to see this applied to other texts, especially those that are ancient and/or canonical (e.g. the Bible, the US Constitution, etc.)–it would be very interesting to see how passages that we take for granted today have been altered through time (think Bart Ehrman’s work visualized).