WordPress is a state-of-the-art semantic personal publishing platform with a focus on aesthetics, web standards, and usability. What a mouthful. WordPress is both free and priceless at the same time.
More simply, WordPress is what you use when you want to work with your blogging software, not fight it.
Serendipity is a Weblog application which aims at giving you an easy way to maintain your own individual diary or personal homepage. It ships with a variety of plugins to plug-and-play with your blog.
Serendipity is PHP-powered and aims for flexibility, scalability, security and ease of use.
It includes all the features of traditional blog tools, and extends them with evolved features such as file & photo management, advanced skinning, multiple blogs support. multilingual as well as detailed user permissions and third party plug-ins.