[Picture borrowed from 'Spot the Differences' PSP game]
*Updated 4/4/2011* Previously I introduced Files Compare Tool for this task, but after some time I found that it lacks certain features and doesn't support Unicode (chinese words for example). Winmerge is the better software overall and it never let me down.
I found this tool extremely useful for programmers when they need to find out what has been modified from the original. This tool will show you the difference between two files (text only) instantly.