Serendipity

[A clip from their web site]

Serendipity is a PHP-powered weblog application which gives the user
an easy way to maintain an online diary, weblog or even a complete
homepage. While the default package is designed for the casual blogger,
Serendipity offers a flexible, expandable and easy-to-use framework
with the power for professional applications.

Casual users appreciate the way Serendipity’s sophisticated plugin architecture
allows you to easily modify both the appearance of your blog and its
features. You can install more than120 plugins with just one click,
instantly enhancing your blog’s functionality. No need to edit code!

Web sites that are using Serendipity that I like:

• Coffee|Code : Dan Scott, Caffeinated Librarian Geek
• Oldham Symphony Orchestra
• Dave’s Serendipity themes