I had been using Google's Image Search to find better versions of photos and to discover names of the photographers etc. for pictures on the Internet since Image Search was announced. It really seemed magical to me when I discovered it and I wished I could submit the many photos I collect and take to scan for duplicates based on image content. Frankly, I thought it would require a shocking amount of processing power and did not imagine a desktop app could pull this off. However, PhotoSweeper does just that.
Usage is silly simple. Just drop a folder into the app's work space, hit compare and evaluate the results. The app will find cropped versions, color manipulated versions etc. for comparison. You can use a slider to make the comparisons more or less picky. And the app can select projects or folders in your iPhoto or Aperture libraries (Lightroom too, but I don't use it) as well.
Once you mark the photos you want to move or remove, just hit the button and the work is done. Databases in the applications are updated reliably as far as I can tell. I have a 50,000 pix library in Aperture and am always careful and nervous when I do something to it on a large scale. Obviously, backup is the word of the day.
The app will sometimes disagree with me on which photo to keep, but I can understand it. For example, I will tend to keep a slightly smaller image if it is less compressed. Using certain combinations of settings PhotoSweeper will keep the larger one. Either way, you can decide to unmark photos or to mark them manually and the settings allow you to control for the elements of photos that are most important to you.