Unix

Diffstat
Makes ASCII histogram from diff output. Download targz source.
http://dickey.his.com/diffstat/diffstat.html

Meld
A gnome2 diff, and merge tool for file comparisons. Edit files in place and the differences are updated dynamically. It also integrates with CVS either as an external tool or via the built in browser.
http://meld.sourceforge.net/

XDiff
Rudy Wortel's X11/Motif based file comparator and merge tool for IRIX, SUN, Linux and AIX. Free, no source code.
ftp://ftp.sgi.com/sgi/xdiff/xdiff/index.html

XXDiff
Graphical front-end for diff. Open source Linux targz, FAQ, screenshots.
http://furius.ca/xxdiff/