The GNU diffutils are comprised of diff, diff3, sdiff, and cmp, utilities for showing differences between files. The manual also documents patch, which uses diff output to update files.
Meld is a visual diff and merge tool. You can compare two or three files and edit them in place (diffs update dynamically). You can compare two or three folders and launch file comparisons. You can
... [More] browse and view a working copy from popular version control systems such such as CVS, Subversion, Bazaar-ng and Mercurial. [Less]
Kompare is a graphical difference viewer that allows you to visualize changes to a file. Whether you're a developer comparing source code, or you just want to see the difference between that research
... [More] paper draft and the final document, Kompare is the tool you need. [Less]
KDiff3 is a graphical text difference analyzer for up to 3 input files, provides character-by-character analysis and a text merge tool with integrated editor. It can also compare and merge directories. Platform-independant.