In which I review Visual Differ (because copying files with finder stinks).
Visual Differ: http://visualdiffer.com
Description from their site:
Visual Differ compares side-by-side folders and files using colors to visually show differences. From the Folder view it's possible to make operations on folders and files like copying or deleting. It's possible to compare quickly directories choosing base folders from the context menu. Using the File Filter feature is possible to hide from visualization specific files like .git, .svn directories or the omnipresent .DS_Store files.