WinMerge

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.
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WinMerge 2.14.0 - latest stable version

WinMerge 2.14.0 is the latest stable version, and is recommended for most users.
Download Now! Version 2.14.0; 6.14 MB

WinMerge 3

WinMerge 3 will be modern compare/synchronization tool. It will be based on Qt library and cross-platform. You can use the same tool in Windows and in Linux.
WinMerge will not be just WinMerge 2 with new GUI. There will be radical changes in how WinMerge works and how it looks. Many features won’t be available anymore and new features will be added.
Read more and get involved!

from http://winmerge.org/