Thesis is Popular SEO optimized theme for WordPress users. Thesis has lots of SEO Features and it is a Do-It-Yourself Theme. I have some personal experience with thesis customizing Thesis is very easy when compared to other Themes, you have to only work on custom.css and custom_functions.php. I have Personal license for thesis and surprised to see Thesis 1.8 Available for download in DIYthemes.
Following are the New Features Available in Thesis 1.8
1. Header Image Uploader
2. SEO Character Counters
3. New Google Fonts!
4. WordPress Nav Menu Support
5. Category/Tag/Taxonomy Page Options
6. Favicon Uploader
7. Custom Loop API
The Price of Thesis Theme will be increased once Thesis 2.0 hits the market