This tool was sent into Cool Tools by Paul Vilarino as a very cool tool.
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.
See also http://sourceforge.net/projects/meld.