Dirdiff is a graphical tool for displaying the differences between
directory trees and for merging changes from one tree into another.
Dirdiff can handle up to 5 trees.  It displays a main window with a
list of the files which are different between the trees, with colored
squares to indicate the relative ages of the versions.  A menu allows
you to display the differences between any two of the versions in
another window.  Another menu allows you to copy the file from one
tree to another.