drupal.org currently carries a story by its author Dries Buytaert where he compares the popularity of CMSes (and blogging systems and forums) using the Alexa traffic ranking service. There's several nice graphs included, so have a look.
Also, there's a similar service called g-metrics.com which can be used to create nice graphs from the number of hits Google returns for a given keyword. See the graph for Drupal for an example.
(via Peter van I. via email and drupal.org)