A colleague of mine pointed me to Xinha Here!, which is a Firefox extension for html text areas. When you’re posting a comment to a blog, you can right-click in the comment field and select the Xinha here! context menu item and you’ll get a full featured WYSIWYG html editor. You can type your comment, add links, do font treatments, etc, and when you hit the ‘OK’ or ‘Apply’ buttons, the text is transformed to raw html and pasted into the comment field. Super handy.
Of course, not all blogs take html, but enough do to make it useful. And it’s not limited to blog comments. Pretty much any html text area is up for grabs.