[[http://www.winmerge.org/|WinMerge]]

Example configuration:

{{{
[merge-tools]
winmergeu.args=/e /ub /dl other /dr local $other $local $output
winmergeu.regkey=Software\Thingamahoochie\WinMerge
winmergeu.regname=Executable
winmergeu.fixeol=True
winmergeu.checkchanged=True
winmergeu.gui=True
}}}