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.
- 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.