WordPressFrom the site:
WordPress started in 2003 with a single bit of code to enhance the typography of everyday writing and with fewer users than you can count on your fingers and toes. Since then it has grown to be the largest self-hosted blogging tool in the world, used on millions of sites and seen by tens of millions of people every day.WordPress offers a service through WordPress.com which lets anybody get started with a free WordPress-based blog in an instant. There is also the option to download and install WordPress itself, and then host your own WordPress based blog from a local machine.
WordPress is usable by anybody, and offers immense flexibility while still maintaining a user friendly approach to website development.
For more information please visit their site; WordPress.org