This is a really interesting tool, which effectively shows what your site looks like in the form of a graph.

The key is:

• blue: for links (the A tag)
• red: for tables (TABLE, TR and TD tags)
• green: for the DIV tag
• violet: for images (the IMG tag)
• yellow: for forms (FORM, INPUT, TEXTAREA, SELECT and OPTION tags)
• orange: for linebreaks and blockquotes (BR, P, and BLOCKQUOTE tags)
• black: the HTML tag, the root node
• gray: all other tags

Found via ProBlogger