Suppose you have a folder full of photos that you have taken. The folder contain many photos such as flowers, birds, animals and all kinds of things.
Are there software that when a user give it a file (say, a photo of a red parrot), it will go to that folder full of photos and list all the photo that look similar to the one that you have provided? (say, it will display photo of parrot of any color and can be a group of parrot or flying parrot?)
As Jack mentioned, it's very hard for software to do exactly what you describe--to "know" what a parrot is and what different pictures of a parrot could look like. That kind of pattern matching may be intuitive to humans, but it's much harder for a computer to think like that.
If you want to search for photos that are "similar to each other" in a way that makes sense to a computer (e.g. different versions of the same photo), try VisiPics. It lets you adjust the search strictness between exact matches and looser matches.
If you have some programming chops you can try your hand at using LibPuzzle, but there does seem to be some software (that requires Adobe Bridge) called Imense that will do this.
Hopefully Google will one day integrate their similar image searching mechanisms from google search into Picasa. For now they have an "Experimental" feature that allows for the finding of duplicate images, but I think that really is meant for duplicates and not for similar images.
