bulletproof comes with 6 basic layouts:
This covers the most popular layout types for websites and blogs: 3-column layouts with various sidebar combinations, both 2-column layouts possible, and an additional 1-column layout showing the sidebars below the actual blog content (the entries). All of these layout are flexible, meaning their column's widths adjust to the width of the viewer's browser window.
You'll notice some of them are marked CF (content first), meaning the actual content of the blog (the entries) is the first item in the output code generated by Serendipity. It would have been possible to make all layouts CF, but that would have required additional HTML markup which we, the developers, felt was not worth the effort.