I was looking for a tool that would allow me to diff two directories last week, so I could try to see what a patch was changing. The top result and a quality product was WinMerge, an open-source application that can find differences between directories and then merge them together if you want.
WinMerge is hosted by SourceForge and has its own webpage at WinMerge.org where you can learn more about the program. To find the differences, you just launch the program and start a new comparison. Select one directory for the “left” and a different directory for the “right”. You can also choose compressed files to compare against and filter things by file name or file extension. When you’re all set hit Ok.
The results may take a minute to calculate depending on the size of the directories and when it completes, it will look like the screenshot below. You can sort by any of the attributes and see which files are different in the comparison result column.
After the comparison, you could elect to merge the two directories and deal with the conflicts where the files are different.
WinMerge helped me see that a patch wasn’t being properly applied, so I had to reinstall the update. Check out WinMerge.