Pretty much exactly what the repo's name says. Features cursor proximity detection, fluid widths, potentially unlimited levels of subnavs, and CSS3 transitions, all without JavaScript (JS is used to smooth over some things for older browsers).
History:
The first build of this project was completed in May 2010; it started as a sort of self-appointed dare to see if it were possible to create a CSS-only dropdown menu with fluid widths and cursor detection (supplanting the need for "proximity testing" scripts - e.g., Suckerfish). After much caffeine and elbowgrease, I actually did it.
The subsequent transition-supporting build was completed somewhere in September 2011; it uses a polyfill (powered by jQuery) to add transitions for browsers that don't support them in CSS.
Support:
Neither of these menus are tested or supported in Internet Explorer 6, and quite frankly, I couldn't give a damn, either. Move on, people.
Slightly bulky and awkward in IE7, but fine on IE8+.
Naturally functions as smooth-as-silk on Firefox, Chromium, Opera, Safari, etc.