Serendipity just released version 1.2.1! It's got several interesting features, but the Serendipity Bulletproof template raises the bar to the professional level. I helped develop the image chooser for bp, so I got a sneak peek.
Bulletproof allows you to modify your blog's template almost trivially. It works in all browsers. It has tons of options, including multiple color sets. Some of the predefined options look really slick. Several developers have been collaborating for months to complete it. In my opinion, it's a giant leap ahead of anything else for any blogging platform.
I'll be modifying my site to use bp soon. Expect some appearance changes!