From my point of view http://themes.weboy.org/ is probably the best site for free wordpress themes. The site is well organized and the themes are sorted into different category. So you can choose easily the themes according to your demand.
WordPress themes based on various category:
2 columns
3 columns
AdSense Ready
Black Color
Blue Color
Brown Color
Clean Style
CMS Style
Corporate Style
Custom Colors
Custom Header
Dark Color
Fixed Width
Flickr Integration
Fluid Width
Free
Gravatar Ready
Gray Color
Green Color
jQuery Integration
Left Sidebar
light
Magazine Style
No Images
Options Page
Orange Color
Photo Style
Pink Color
Premium
Purple Color
Red Color
Right Sidebar
Rounded Corner
SEO Friendly
Simple Style
Square Corner
Sticky Post
Threaded Comments
Threaded Comments Ready
Twitter Ready
Valid CSS
Valid XHML
Web 2.0 Style
White Color
Widget Ready
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