Wordpress is a blogging tool and weblog software with good support from the open source community

Website: Wordpress.org

Community: Wordpress.com community

Templates: Wordpress themes | Themesbase | Weblogtoolscollection
Plugins: Wordpress plugins | wp-plugins.net - big collection of wordpress plugins

Demo: Wordpress Demo

Download: Wordpress Download Page

Requirement: MySQL / PHP 4.2 or greater / Apache or Litespeed

Features

Wordpress is one of the most widely used blogging tool in the internet. It has a big community to support its development. The code has full compliance with W3C standards.

Switching between multiple templates is very easy. Templates and permalink structure are the best for SEO. Fully supports the Trackback and Pinging standards. Wordpress supports pinging through ping-o-matic.

Individual posts can be password protected. Comments can be moderated automatically using Akismet.

Wordpress has hundreds of plugins to support you for all of your requirements. It has good support for external tool integration.