http://www.smashingapps.com/2008/08/20/21-mindblowing-premium-like-free-wordpress-themes.html Check them out, I thought they were really cool. I especially like the Mimbo magazine one, looks really pro/clean and un-blog like.
Mimbo has been around for a while, but that "Options" theme by Justin Tadlock is pretty sweet ! I have paid good money for similar themes in the past.
Yeah, actually most of the themes listed has been around for quite some time. But good things never turned old, without doubts those are well-designed themes that worth for the collection. But then again, my favourite WP theme is still the one done by Revolution. (I cant post URL here but it's on revolutiontheme dot com, go see yourself!)
here's another cool one: http://www.smashingmagazine.com/2008/09/08/agregado-a-free-wordpress-theme/ in fact, i'd recommend spending some time on Smashing Magazine, it's one monster of a resource for web people (and designers).