HTML5 now has an official logo, looks oddly reminiscent of Superman's leotard
Neither HTML4, HTML3, or indeed any version of HTML, has ever had its own logo before. There is an ancient 'HTML square', but as that dates to before 1995, it probably accompanied HTML2 and appeared on just a handful of websites.
The HTML5 logo website itself is, rather fittingly, a beautiful bastion of pure semantic goodness. Gone is the awful Web 2.0 'feedback' tab, with a beautiful 'Nav' menu sitting its place. There are also great descriptions for each of HTML5's major new features and some nice 'badge designers' -- in case you want to put the HTML5 logo on your site.