Meld is a powerful visual diff and merge tool. It displays colour-coded two- and three-way diffs and enables you to merge or edit the compared files. It can work with version control systems (cvs, svn, hg) and can diff directories. The GTK-based interface is clean and features tabs and a toolbar.
Meld does a great job of simplifying edit and merge tasks; when comparing files, for example, you can simply click on the arrows in the middle column to merge text from one side to the other (see screenshot). Holding down Ctrl reveals insert-before/insert-after options, and holding down Shift enables one-click block deletion.
Installing from the command line: yum install meld
Menu location after installation: Applications > Programming > Meld Diff Viewer
Upstream website: http://meld.sourceforge.net/