CloneSpy is one of those indispensible tools, one of the ones that I install first on a clean-install of Windows and never get rid of. What it does is take checksums for a bunch of files and compares them to find duplicates.
One of the useful things is the “pools,” in which you can take two seperate sets of folders and compare them against each other, and sometimes automatically delete the duplicates from one of them. For example, when I do a clean-install, I usually tend to restore all my data from a backup. And sometimes I’m reluctant to delete the Windows.old in case I accidentally delete a file I haven’t backed up. So what I do, I put the User folder into pool one, Windows.old into pool two and set it to automatically delete, then go take a bath. When I come back, the leftover files are still in Windows.old, so I transfer them over then delete.
Also especially useful for duplicate pictures. One limitation in VisiPics is that even on “strict” setting, it sometimes grabs pictures which are very similar, but not quite. (I’m a bit of a hoarder.) So by checking the checksums, it can find the exact same one.
Oh and it’s free. 🙂 Free is always good with me.