WordPress
ThoughtStorms Wiki
The leading, and very good, FreeSoftware BloggingSoftware.
I started using WordPress for a bunch of my small scale sites but PHP, or some of the plugins, are insecure, and I've pulled back to making static sites in BootDown.
I still use WordPress for the SmartDisorganized blog : http://sdi.thoughtstorms.info/
WordPress creator MattMullenweg and his company Automattic are pretty cool. Very dedicated to FreeSoftware (under the GnuPublicLicense), running as a truly distributed company, and have good commitment to diversity.