A free alternative to Changes (for Mac OS X) and Beyond Compare (for Windows) is FileMerge for Mac OS X. It is a free utility that comes with Apple Developer Tools. FileMerge can be launched in the Terminal (command line) by typing
opendiff dir1 dir2
FileMerge differs from many comparsion utilities because it doesn’t show each directory in two side by side panes. It shows the file differences in a single tree hierarchy.
FileMerge, Beyond Compare, and many others are compared on the “Comparison of file comparison tools” wiki page