I have been exploring a new open source content management system called MODx. Its designed to be extremely flexible. It is aimed at developers who like to their hands dirty. Its admin interface is based on AJAX so that even though it looks as good as joomla its actually a lot faster.

While other CMSs provide limited support for modification  MODx embraces cutomisation. You can specify your own CSS, insert your own HTML code and your own PHP and javascript snippets. You can then insert MODx tags in the appropriate places to add content to the HTML files.

I tried it out at opensource CMS and I think it is probably the most flexible CMS I have tried yet. Because it is so developer oriented the learning curve can be a bit steep. And of course, you must really have a need for such low level control of your website. MODx also does not seem to support any sort of remote posting API such as blogger.

From a purely development point of view I think this CMS stands out from the crowd.