Seth Gottlieb has written an excellent article on open source CMS', published in January of this year by Optaros. Without the descriptions of example of the individual open source CMS', the document would work very well as a standalone generic introduction to evaluating not only whether (and why) to choose open source, but how to choose the right CMS for the type of website you need.
Deanne has an excellent summary of the article, and to the list of suggested improvements I could add that Drupal could also fit under the "Dear God, this can do everything" category. At my employer's weblog, there was once a "Drupal Does This" category (though my brief article remains, as does Boris' 'Drupal Does This' post on his own site), as it can do brochure sites, news websites (if a satire site can be considered news), and weblogs (like the one you're reading), as well as, of course, community sites. True to its motto, the flagship Drupal site turns out to be one of the better examples of latter type of site.
(Some readers, especially those who watch my del.icio.us links, will no doubt have noticed—and possibly tired of—my promoting Drupal quite a bit lately. It's actually my way of keeping track of developments with not only the software but how an open source project can be organized and maintained. In conversation I've been using it as my only example of how an open source community develops software, I probably need a little more exposure to how other projects' go about creating, maintaining and supporting their software. It would probably help, though, if I knew how to code in something other than PHP.)
In fact, it may be unfair to many of the rest of the CMS' listed in the article, since many of them have frameworks for creating modules or plugins or whatever they call the code that extends and/or builds on top of it. Matt linked to an article suggesting features don't matter, which, in the comments to Matt's link, I say "make it a platform, and people will build just enough features for it." WordPress comes very close to having just enough features for use as a blogging tool, though Matt is careful—and correct—to point out that people are using it for more than just blogging. Drupal is definitely overkill for those that want a weblog. That said, given a set of requirements and use cases (guess what I'm reading up on next?), you can make any modular CMS do what you need it to.