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."
http://meld.sourceforge.net/.
Alternative tools (see meld page) :
Kdiff3, Xxdiff, TkDiff, GtkDiff.
