Clay revolutionized WordPress theme development with the Headway Theme Framework. This was, and still is, the only theme framework with a full real-time wysiwyg visual editor.
Although this makes it easy for anyone to design their own WordPress theme, Clay has made Headway a very powerful tool for developers as well, with an extensive array of hooks and intelligent declaration of CSS selectors.
Version 2.0 of Headway brings a huge array of enhancements and optimizations, all coded by this brilliant up and coming developer.
Headway and Clay are certainly ones to watch in the WP community.