monline

Drupal is the open source (GPLv2) content management system (CMS) of my choice. It's very well written (nice internal structure imho) and is pretty amazing in its modularity/extendibility. Initially it seemed as a late starter (since a lot of other similiar software already existed for years), but nowadays it has caught up with the competition. I've already used it for my blog and a number of company websites, newsportals, etc. It's not the perfect out-of-the-box solution for every purpose, but it's a PHP portal developer's dream.