Also, see Wordpress[0][2] and Vanilla Forums[1][3]. They both offer an open source version that you can download and self-host, and a commercial offering which they host and you can customize through a limited set of themes and plugins.

Open source, community edition:

[0] http://wordpress.org/

[1] http://vanillaforums.org/

Hosted version with customization via themes and plugins:

[2] https://wordpress.com/

[3] http://vanillaforums.com/