OpenSourceCMS
Try out Content Management Systems online - only PHP/MySQL based!
The CMS Matrix - The Content Management Comparison Tool
Tool for comparing CMS (open and closed source) based on selected criteria.
CMS Watch Report
Commercial report, comparing 32 CMS. Parts of it are free for download.
Optaros - White Papers: Service Oriented Architecture and Open Source Solutions
Free white paper: Comparison of 16 Open Source CMS