I agree that the open-source thing is pretty awesome, but I was rather disappointed at WPMU, the multi-user interface I never quite got to work properly. Not to mention the templates seem to have more files than I'm used to hacking. XD Still, WP has good features and a nice, clean interface.

Wordpress is an EXCELLENT blogging platform. While their hosted solution is somewhat limited they offer their software as open source so you can install it on your own server and even edit the source code to add your own functionality! Great for the advanced blogger.