xxdiff is a free graphical diff program.  Its home page is here.

	http://xxdiff.sourceforge.net/

xxdiff is descended from xdiff, and xdiff is descended from an old
OpenGL program from SGI called gdiff.

xxdiff's coolest feature is the rightmost pane, which conveys a lot of
information about the files' differences and your view in a small
area.

xxdiff allows a lot of manipulation via keyboard equivalents -
movement by lines or regions, selecting/deselecting lines, regions, or
globally.  It's a strange oversight that it doesn't have a keyboard
equivalent for "Redo Diff".

Image xxdiff-1.png shows a somewhat contrived diff of a short file.

Note the rightmost pane, which shows where the current view is in the
two files, and where all the differences are in the files.  Also note
the triangular ticks which show the current line in both files.

The color coding is...

	gray	  - these lines are identical in both files.
	yellow	  - these lines are different in both files.
	green	  - these lines are only in one file.
	magenta   - these lines are selected for the output file.
	dark gray - these lines are deselected for the output file.