Sure the built in new version of Word Pad, which comes with most if not all versions of Windows 7 supports highlighting text.
As far as email programs, I know
all support it, not sure about all the others.
If you want the text highlighted "in situ", that is within the program which generated it (as in email within Eudora) then you're only going to get that if it is a feature of the program itself. Highlighted text is a variation of the font supported by the application so it requires the application itself to support it. I'm not aware of any external app which will modify the text appearance which is displayed by another app.