There is something similar built into OSX, but it just wouldn't solve my problem.

I had 3 iTunes libraries on internal and external drives. Some songs were in all three libraries. Some songs were in two of the libraries and some were in only one, but it could be any one of the three. Two of the libraries had hundreds of songs and the third had thousands. The thought of sorting out this chaos manually was a nightmare. Compare Folders to the rescue.

I decided to pick one as the master library and compare the two other libraries in turn. You can view duplicate, unique or similar items and so using the Copy To function you can soon get the chaos into order.

I could have done the same job with one of the synchronisation apps that I have, but the real benefit of Compare Folders is the way that what is happening is laid out so well.

Criticism? No family licence available. With soon to be 4 Macs in the house, this would have been a nice option.