WordPress site-styling is template driven, changing the way your sites look & feel is as easy as finding a new style FTPing it to your themes directory and enabling the style from withing WordPress.
Here is a list of theme resources I have found useful
Smashing Magazine Post – 83 Beautiful WordPress Themes You Probably Haven’t Seen
Smashing Magazine Post – 100 Excellent Free WordPress Themes
WP ThemeSpot
Top WP Themes