Kaleidoscope is an application for the Mac that allow you to compare different versions of text, folder and images. Their motto is: "Whatever you write, your first draft is never the last. Spot changed text quickly and merge instantly."
The application costs $69.99.
ConnectedText offers such a tool natively:
It is restricted to text, but I would suppose most people who have use for Kaleidoscope would use it mostly for text as well. In any case, the ability to compare different versions of the same topic has come in very handy several times during the last few years. I would be interested in Kaleidoscope, if I did not do most of my writing in ConnectedText.
1. By way of David Bosman.