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The Thesis Themes are some of the most popular in the blogosphere and are easy to modify. The themes are created so even if you only have a limited knowledge of CSS and HTML, you can tweak the Thesis theme and create an awesome, unique design.

Natty WP Premium Themes offer a mix of some very good themes with a lot of very ordinary themes that shouldn't be considered Premium.

WP Zoom offers a wide variety of Wordpress Premium Themes, with an emphasis on magazine style themes.

Studiopress make a variety of Premium Themes (12 at the time of writing). They have a pretty good deal where you can purchase all themes at one price which also comes with the accompanying support.

Mimbo Pro Premium Theme is one of the better supported Premium Themes. It is a magazine style theme.