The Drupal people produced insightful stock-market-like statistics about the popularity of open source CMS packages (via the precious Amphi-Gouri). But their analysis mixes content management systems (Drupal, Plone) with blog engines (WordPress) and bulletin boards (phpBB). Anyway, it shows that :

• « The popularity of most Free and Open Source CMS tools is in an upward trend.« 
• Bulletin boards like phpBB is the most popular category, maybe the most mature and phpBB is the strong leader in this category
• In the CMS category, Mambo, Xoops, Drupal and Plone are direct competitors ; Mambo is ahead in terms of popularity, Plone is behind its PHP competitors which certainly benefit from the popularity of PHP compared to Python; PHP-Nuke and PostNuke are quickly loosing some ground.
• WordPress is the most dynamic open source blog engine in terms of growth of popularity ; its community is exploding

My conclusion :

• if you want an open source bulletin board/community forum, then choose phpBB with no hesitation
• if you want a real content management system and are not religiously opposed to Python, then choose Plone, else stick with PHP and go Mambo (or Xoops ?)
• if you want an open source blog engine, then enjoy WordPress

If feel like producing this kind of statistical analysis about the dynamics of open source communities is extremely valuable for organization and people considering several open source options (cf. the activity percentile indicated on sourceforge projets as an example). I would tend to say that the strength of an open source community, measured in term of growth and size, is the one most important criteria to rely on when choosing an open source product.

Nowadays, the (real) stock market relies strongly on rating agencies. There must be a room (and thus a business opportunity) for an open source rating agency that would produce strong evidences about the relative strength of project communities.

What do you think ?