I've only used databases minimally with my sites. On my business site, I only use one database to keep track of my sales for xBlog. On my personal site, I do not use them at all, and on the Web Dev FAQs site, I should have used one, but did not. :p One of these days I'll have to go back and make a CMS for it. I have, however, used databases extensively on some of my clients sites, and for xBlogPro, which is coming soon.
However, I do use PHP includes on nearly every site that I make (all three listed above) as it makes it incredibly easy to keep navigation current across the entire site.