Content management specialist Seth Gotlieb has written an easy to understand how-to on selecting an open source CMS at zdnet. Gotlieb is also responsible for the whitepaper ‘Content Management Problems and Open Source Solutions‘ which summarizes 15 open source projects and distinguishes between open source CMS and proprietary software selection.
Many Companies want a formal tech support solution where they can call someone on the phone to ask questions and blame. I think many of the smaller open source projects are over looked because of the lack of phone support; however, some of the larger projects such as Open Office do provide tech support and are used by more companies because of it.
Good place to start looking is http://www.opensourcecms.com/