The Valid XHTML user script is an adaptation of the blogidate XML well-formedness bookmarklet. It shows a line of text under each textarea indicating whether the text is well-formed XHTML. When the text is not well-formed XHTML, it displays Gecko's error message and gives you a link that selects the location of the error in the textarea. When the text is well-formed XHTML, it displays links that let you check whether the XHTML is valid in addition to being well-formed.
By default, it only runs on admin posting pages for Movable Type and WordPress and on archive pages for Simon Willison's blog. You can use Greasemonkey's interface to make it run on the sites on which you edit XHTML.