MD5 File And Folder Comparator Lite ($16; free, feature-limited demo) is a useful tool for doing two slightly-different tasks: Verifying that two versions of a single file are byte-for-byte identical, and determining what differences, if any, exist between two folders. Both of these functions are useful for security and especially for managing multiple versions of software.
The interface is not the prettiest, but it gets the job done. There are no deeply nested dialogs full of tabbed options; everything you can do with MD5 File And Folder Comparator Lite is right out there in the open. This can be slightly confusing because you have several different tasks all on the same page. For example, you compare two files on the top, and two folders on the bottom, but there's no necessary relationship between the two functions.
"MD5" is a type of hash, meaning, a code created by applying a mathematical formula to the bytes of a file. It is almost impossible for two different files to have the same hash signature. This means that even if two files have the same name and the same size, you can detect differences by comparing their signatures. This is very useful for checking that a file hasn't been tampered with, or just that in the chaos of making multiple builds, Version 188.8.131.52 didn't accidently get labeled as Version 184.108.40.206.
The other half of MD5 File And Folder Comparator Lite--one which will probably see more common use--is the ability to compare the files contained in a given directory. "Eyeballing" two directories full of similarly-named files is rarely going to catch all the differences. MD5 File And Folder Comparator Lite can produce a report showing which files are missing, added, or changed in one directory as compared to another, and will also delve into subdirectories. The demo version is limited to 100 files.
MD5 File And Folder Comparator Lite can be a useful tool if your need for this type of program is sporadic. If you perform these checks on a regular basis, you may want to look at the non-lite version, which includes features to save and load the signatures.
Note: This demo version can compare only 100 files at a time. There are no limits on the full version of MD5 File And Folder Comparator Lite, nor on the $24 MD5 File and Folder Comparator.