This might be well-known, but I just discovered it now. I wanted to copy some text from a Web page to an email message, so I copied the text in Safari and pasted it into a mail message. The pasted text included all sorts of hidden junk, such as comments, javascript and extra line breaks. So, I dragged the text to the desktop to create a text clipping and then dragged that to my email message. Voila! Clean, plain text.
Mac OS X Hints
http://hints.macworld.com/article.php?story=20031106131246622