WordPress.com has introduced a news Theme Showcase where you can browse and search all themes available on WordPress.com.
Themes can be browsed by subject or other criteria.
“The Theme Showcase exhibits the amazing collection of themes available on WordPress.com, documents theme features, and gives you a central place to explore themes outside of your dashboard. With colorful screenshots, detailed specifications, links to sites using the theme, and a list of suggested uses, I know you’ll be inspired to learn more about your current theme or try something new.”
More info: http://theme.wordpress.com/