WinMerge
Open source differencing and merging tool for Windows
Meld
Visual diff and merge tool for files, directories and VCS projects
KDiff3
File and folder comparison and merge tool
Araxis Merge
Two- and three-way file comparison, merging and folder synchronisation
Diffchecker
Online and desktop comparison of text, images, documents and spreadsheets
difftastic
Structural diff that understands syntax