What is Textpattern?
Textpattern is a content management system created by Dean Allen. Written in PHP, it was designed to be used with all kinds of web sites, including weblogs. Built into Textpattern is Textile, a simple syntax for nudging plain text into structurally sound and stylistically rich web content. Textpattern has been released under the open source GNU General Public License as of version 1.0rc3.
Plenty more useful information in the FAQ.