Diff'nPatch is a GUI application for the UNIX diff and patch commands. The diff function allows you to see what differences there are between two documents, folders, and more. It's AppleScript compatible.
What's new in Diff'nPatch
Version 1.4 adds the following:
Select in the Finder two files you want to compare and just drag and drop them on the icon of the application.
drag and drop a file from the Finder to the appropriate text fields in the dialog windows instead of using the corresponding "Set" button.