Google trends is a tool that analyzes Google web searches and that can visualize search trends over time using so called "search-volume graphs". These graphs usually provide a good mechanism to compare the popularity of two or more products.
Here is the search-volume graph that compares Drupal and Joomla:
It is worth pointing out that Joomla has been around a lot longer than the graph suggests. In 2005, the bulk of Mambo's core developers left Mambo and started Joomla after a dispute with Miro Corporation, the company that founded Mambo.
