sudo apt-get install meld
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 browse and view a working copy from popular version control systems such such as CVS, Subversion, Bazaar-ng and Mercurial. Look at the screenshots page for more detailed features.
sudo apt-get install kompare
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 paper draft and the final document.
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.