I'm presently looking into blogs that offer some different functionality than presently available in pMachine Pro for a colleague and I came across this very useful comparision chart by Owen.
In addition to this very useful chart, Owen has links to the OpenSourceCMS site which hosts demo versions of various php/mysql CMS applications including the likes of Drupal, pMachine free (the free version of this blog), Moodle, ACollab etc.
The site allows you admin access to the systems so you can get a reasonable look around (a few functions have been disabled for security reasons). The system is then cleared down every 2 hours.
I think this is a great site for doing some of that initial evaluation into a potential CMS/Blog/Discussion Forum/etc candidate - I only wish I found it earlier.
And if you want to find out which blog Owen finally chose, you can view his blog entry on his decision here.