While I totally agree with & upvoted the accepted answer (upvote), here are a few gratis alternatives:
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.
You can also choose a tree view of the same information:
KDiff3 is a diff and merge program that
•compares or merges two or three text input files or directories,
•shows the differences line by line and character by character (!),
•provides an automatic merge-facility and
•an integrated editor for comfortable solving of merge-conflicts,
•supports Unicode, UTF-8 and other codecs, autodetection via byte-order-mark "BOM"
•supports KIO on KDE (allows accessing ftp, sftp, fish, smb etc.),
•Printing of differences,
•Manual alignment of lines,
•Automatic merging of version control history ($Log$),
•and has an intuitive graphical user interface.
•Windows-Explorer integration Diff-Ext-for-KDiff3 - shell extension included in installer (originally by Sergey Zorin: see also Diff Ext)
•KDE-Konqueror/Dolphin service menu plugin
•Simplified integration with IBM-Rational-Clearcase for Windows
•Any Un*x that is supported by the Qt-libs from qt-project.org,
•Apple Mac OSX binary available. (0.9.98)
•In theory any platform for which Qt-libs work (Qt4 or Qt5).