OSCOM is the international association connecting users and developers of Open Source Content Management solutions.
CMSWatch is an independent source of information, analysis, and reports about web content management solutions.
Grow Your Own (GYO) is Babel's own attempt at a CMS. Still in its early days. Aims for simplicity rather than features. It's good for doing a simple 2-layer site with easy editing (wiki based) and simple navigation. Has simple database integration but doesn't have lots of other features.
An Australian designed engine, with many features. Has a good WYSIWYG page editor and has table layouts on every page. Template design is complex, set up and installation is complex. PHP based, with a good code base, object oriented and easy to extend. I like it for large sites, overkill for small ones.
This one I'm not fond of. The content management framework binds into Apache as a loadable module and takes over from PHP. Code is stored in the database, which makes code editing and version control very complex. Also, because of the snippet based code base it can actively discourage object oriented code.
The one used at http://www.osia.net.au/. I find it relatively easy to navigate but has some bugs. It also doesn't like most HTML tags posted into the page documents but doesn't have a good wiki based or WYSIWYG editor to get around that, so page editing you're left fumbling a bit. Not good documentation.
Kupu is a 'document-centric' open source client-side editor for Mozilla, Netscape and Internet Explorer. Kupu can be easily integrated into a variety of CMS.
Allows you to do side by side comparisons of many CMSes.