WordPress is the most popular blogging platform out there. WordPress has lots of great looking themes available for free. Serendipity is also a great blogging platform but it does not have the same range of excellent themes or templates that WordPress does.
It has been a long time since I created a template for serendipity (s9y). BlackLens is an original two column template with a horizontal top navigation bar, customizable footer and a fancy header.
I am releasing the Nightwatch serendipity theme for free. I call it that because the theme looks good if you set a night time cityscape as the header image. You can see a preview of it below: