Greetings to my new friends here at DC. As a photo enthusiast, I have enjoyed this thread and its many useful recommendations. I thought it worth noting that both the previously mentioned Visual Similarity Duplicate Image Finder and PhotoSort have demo versions which work quite well with only minor limitations. Specifically, neither allow moves/deletes but otherwise work just fine without any time limitation. Of course, if you can afford to register and get full functionality, then by all means...
Likewise, FirmTools' Duplicate Photo Finder at
http://duplicatefinder.firmtools.com/ offers what they characterize as a "trial" version of their product. To me, it's a demo version like the others mentioned because it functions without time limitation but cannot perform moves/deletes. This Firmtools software IMHO needs a similarity/tolerance adjustment like Visual Similarity Duplicate Image Finder and PhotoSort have, but it does utilize cache and process quite quickly. If you just want to find the most similar pictures in your collection without a lot of fuss, Firmtools' solution might be your cup of tea.
Finally, I didn't see a mention of XnView in this thread. Among its myriad attributes, this excellent freeware image viewer at
http://www.xnview.com/ works amazingly well in finding similar images via "Tools/Find similar files". With its adjustable similarity/tolerance level, it doesn't get much easier to process an image collection.