This is a nice looking WP theme with the following features:
-Space for 121x121 or similar sized square ads.
-Nicely styled calendar for use with widgets
-Widget ready sidebar and footer
Demo: http://demo.themelab.com/index.php?wptheme=Nature+Blog
Screenshot: http://www.themelab.com/images/nature-blog.jpg
Download: http://www.themelab.com/downloads/nature-blog.zip
Link: http://www.themelab.com/2008/04/16/nature-blog-free-wordpress-theme-49/
Designer and coder links in the footer.