I chose the script tag for this one. I am assuming this is a tag that inputs javascript, but have been seeing it all over the web and thought it would be a good one to discuss.
I haven't seen the iframe tag very much across my web searches. It seems to be for videos, but thought it was pretty cool.
I was looking through the source code on my rock gyms site and noticed a td tag. I have no idea what this would be for besides displaying text with the ability to style it within the element.