Tool for comparing a single line of text
I use vimdiff often to compare two files side by side. vimdiff in color mode will highlight all differences between corresponding lines on each side. I'm using it to look for different characters on two different lines (in different files) right now.
kdiff is one open source diff utility that shows character by character differences as well as line by line. I'm sure there are others. Here's link for kdiff: http://kdiff3.sourceforge.net/
DiffMerge does pretty good at highlighting word or character differences. Better than WinMerge, IME.