Where to find unusual themes?

Looking through available WordPress themes, I see that what is called a theme is usually a change of window-dressing, not structure. Mostly just makeovers – the colors changed, header image changed, but still 1,2,or 3 columns, a header, footer, & so forth. WordPress blogs mostly look the same. Where can I find a list of themes in which the designers have done something unusual with the structure of WordPress, and not just redecorated the same old design?