inove is one of the most popular themes that are available on wordpress website. It has thousands of downloads and most users rate this as excellent skin for their wordpress blog. The theme is universal - it can work for many websites, not only for blogs or personal homepages.
The theme has been taken from wordpress site and then converted into concrete5 skin as contribution to c5 community. It's popular on wordpress, so maybe it will have some success among concrete5 users as well.
'Rebuild' is a flexible and highly customizable theme. Most aspects of the theme can be changed through the 'Customize' feature under 'Pages and Themes' in the conrete5 dashboard. You can change the color of most of the theme elements, as well as change the typography without even touching the css files.
Illustrative is another free concrete5 theme we'd like to contribute for the concrete5 community. The theme has animation movie style, very nice and bright. For title in the sidebar, use h2 style. For title in the main area, use h1 style.
Whitespace is a free minimalistic 3-column concrete5 theme by c5mix and Brian Gardner. Whitespace is distributed under a GNU General Public License, which allows you to use and modify it for any purpose (personal and commercial), under the condition that you keep the provided credit links in the footer.
Shades of Blue is a nice two-column skin, which been originally designed for Wordpress by studiopress.com and converted into concrete5 theme by Smart Web Projects. We like its navigation and the layout in general.
Eos is another theme we converted from wordpress template. We choose it to be the next theme, because we like its modern look-and-feel. Rich gradients will make your website allow, but serious enough even for business websites. This theme is search engine friendly. Valid XHTML 1.0.