Here I thought I had seen every (valid) HTML tag that existed, until I was reading some articles on the W3C site and stumbled across the <FIELDSET> and <LEGEND> tags.

<FIELDSET> groups form fields into a 'container by wrapping a border around all of the fields.  Obviously, you could put anything you want in the Fieldset tags, but it's designed for grouping form fields.

The <LEGEND> tag simply puts a label on the border of the <FIELDSET>.  Here's an ugly sample (which could be made pretty with some CSS):

Internet Explorer 6, FireFox 1.1, and Netscape 7.01 all recognized this tag, so I guess that means it's 'fair-game'!