BleepingComputer Review:

WinMerge is a text file differencing tool for Windows. It can highlight and reconcile entire directory trees of files, showing you exactly what has changed and where between any two versions of the file or directory tree being examined.

WinMerge is optimized for Windows permitting high-throughput for faster differencing.

Product Description from The WinMerge Team:

WinMerge is an Open Source differencing and merging tool for Windows. WinMerge can compare both folders and files, presenting differences in a visual text format that is easy to understand and handle.

WinMerge is highly useful for determining what has changed between project versions, and then merging changes between versions. WinMerge can be used as an external differencing/merging tool or as a standalone application.