Simplish

In no particular order, some things Simplish does for your WordPress site:

• Valid XHTML 1.0 Transitional output, styled with Valid CSS 2.1.
• Title elements are Google-friendly out of the box, using the Pagename - Sitename convention.
• Microformats support: Entries are hAtom microformatted, including ISO published date, author hCard.
• Author archive bio pages with author URL and bio from profile, author Gravatar, wrapped in an hCard. Rather handy on multi-author blogs.
• Threaded comments (aka nested replies) (WP 2.7+).
• Commenter Gravatars.
• Individual permalinks for Comments.
• Nifty Sitemap page template.
• Standard-issue and valid Archives and Links (aka Blogroll) page templates.
• Sensible styling for base elements, often courtesy the Blueprint CSS framework (e.g., tables, lists).
• Simple CSS for print media type, and your readers’ printers.
• Only one image, 256 Bytes (the header shadow gradient).
• Built-in custom image Galleries filter WordPress’s gallery shortcode to output valid XHTML, styled with the theme stylesheet.
• Image Captions styles.
• Stylesheet includes .alignleft | .aligncenter | .alignright convenience classes for content image alignment.
• Tags Taxonomy.
• Choose-your-taxonomy: Use categories, tags, both, or none. Simplish shows what exists; gracefully suppresses Category/Tags display if they don’t exist, per-post.
• Sidebar widgets supported.
• Honors your date format settings.
• Prev/Next post links on single-post views.
• Source is available and free, licensed under the GPL. You can use, modify, and even redistribute your own version of Simplish.
• Listed in (and meets the requirements of) the Official WordPress Themes Directory.