*NOTE: This developer has closed down as of November 2012.
"Compare Folders is a powerful utility designed to compare two folders and tell you which files are the same, which are new, and which have been modified."
Perhaps you've noticed or become victim to a "feature" of Finder: when copying or moving folders from one location to another, if a folder with the same name already exists, its contents are obliterated by the replacement.
This is especially disconcerting for "switchers" as the default Windows method is to merge and only overwrite duplicate files after prompting.
With Compare Folders you can quickly identify duplicate files, unique files (missing from one side), and close matches. The results are recursive, and files can be inspected or copied to a target folder in batch or singularly.
There is no "quick-sync" button to automatically synchronize the two folders and query on conflicts, but it is far and away a time saver compared to Finder.
The demo is limited to 9 compare operations, and offers integration with .Mac (iDisk), Growl and Path Finder. Folder presets can be saved, and results can be exported as text.