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
diffoscope
In-depth comparison of files, archives and directories