Wordpress is the most commong blog software on the net today. It's ease of installation and use makes it possible for almost any webmaster to have a blog site, and it's simple format for writing new entires makes it very easy to maintain and update.
* Wordpress Software - You can download the software on their main site.
Wordpress is also very easy to change the appearance of, using skins that change the appearance of the site without requiring programming skill. It isn't the simplest thing to learn, but the results can be rewarding.