A blog (a contraction of the term "Web log") is a Web site, usually maintained by an individual with regular entries of commentary, descriptions of events, or other material such as graphics or video. Entries are commonly displayed in reverse-chronological order. "Blog" can also be used as a verb, meaning to maintain or add content to a blog.
Many blogs provide commentary or news on a particular subject; others function as more personal online diaries. A typical blog combines text, images, and links to other blogs, Web pages, and other media related to its topic. The ability for readers to leave comments in an interactive format is an important part of many blogs.
http://en.wikipedia.org/wiki/Blog

Textpattern is an open source content management system unlike any other; it allows you to easily create, edit and publish content and make it beautiful in a professional, standards-compliant manner.

The goal of the LifeType project is to create a stable multi-user and multi-blogging platform, to strengthen the concept of communities around blogs.

Serendipity is a weblog/blog system, implemented with PHP. It is standards compliant, feature rich and open source (BSD License). Serendipity is constantly under active development, with a team of talented developers trying to make the best...

DotClear was design only for weblogs management, and do it well. It is completely free! DotClear is a free software distributed under the GNU General Public License.

Nibbleblog is a powerful engine for creating blogs, all you need is PHP to work. Very simple to install and configure (only 1 step).

FlatPress is an open-source standard-compliant multi-lingual extensible blogging engine which does not require a database to work. You don’t need MySQL because FlatPress stores all of its content on text files. All you need is a PHP-enabled web space.

A CakePHP powered CMS

While there are a number of technical reasons that highlight the differences and advantages Habari has over other blogging packages, a major component of what makes Habari different is its community participation model.

SweeRice is a simple website management system ,it is not just free and open source, it provides popular functions such as SEO. Itself enough create common blog or website, of course, you You can also install plug-ins to extend it .

PRITLOG is an extremely simple, small (< 500K uncompressed) and powerful blog system. It does not use or need a MYSQL database and fully works based on Sqlite.

Anchor is a content management system, built especially for art-directed blogs.

LiveStreet is a free engine for blog and social networks.
The project has been implemented with the object oriented programming on PHP5 + MySQL5 and is built on the principle of the MVC model.

Chyrp is a blogging engine designed to be very lightweight while retaining functionality. It is powered by PHP and has very powerful theme and extension engines, so you can personalize it however you want.

doorGets CMS is very easy to use, written in PHP with MySQL Database. You can't get any easier... try it !

HTMLy is an open source databaseless blogging platform prioritizing simplicity and speed (Flat-File Blog). HTMLy using markdown file to store all its posts.

The lightweight blogging system, true for small projects.