I have the default composing format of Mail.app set to Plain Text in the preferences. So if I ever need to straighten up a chunk of crazily-formatted text (removing font, size, bold, color, links, etc.), I hop over to Mail, make a new message, paste the text, then copy it back out. It's worked well for me.
[robg adds: You could also use TextEdit (in plain text mode), or any other pure-text editor, to do this. This older hint shows you how to do the same thing using a one-line AppleScript.]
Mac OS X Hints
http://hints.macworld.com/article.php?story=20060728001843904