B. Mann Consulting

Right now, many of these systems suck for one reason or another.

The most popular (phpnuke, postnuke) suffer from too many developers with too much bad code, as well as forking the code to provide derivatives that aren't much better.

Other systems aren't in widespread use, so have a limited number of easy-to-install modules.

Drupal
This is what runs this site. On my old host, I was running phpWebSite, then switched to postnuke when I moved, then neglected postnuke when I became enamoured of using pMachine for my personal blog.

While Drupal is technically a portal / community system, you can turn things off (like moderation) that will automatically make anything you post end up on the front page, much like a standard blog.

It's theme engine is relatively simple, allowing for maximum flexibility. And, if you remove the CSS code into a separate file and tag individual items appropriately, almost all the layout / presentation can be done using CSS.

There are a few add-on modules, but since Drupal went to 4.0, not all are ready for prime time yet.

Ampoliros
Calls itself a web application platform. Looks very clean, and offers the excellent option of installing/upgrading modules completely through a web interface, including upgrading of the base application.

PostNuke

phpWebSite