I use WooThemes as well.
While it's not a Plugin per se, this is a cool tool I picked up:
http://www.artisteer.com/
Artisteer is the first and only Web design automation product that instantly creates fantastic looking, unique Website templates and Blog themes.
Design awesome Wordpress Blogs and professional Websites in Minutes
Easy to Use
No need to learn Photoshop, CSS, HTML or other technologies
Export as Wordpress Theme or CMS Template
With Artisteer YOU immediately become a Web design expert, editing and slicing graphics, coding XHTML and CSS, and creating Web design templates and Wordpress themes - all in minutes, without Photoshop or Dreamweaver, and no technical skills.