Some three years ago, the nice people at Drupal launched a service that gathered statistical data from deployed drupal sites, regarding the modules they used. This was mainly to solve the problem of which module to use for a particular purpose.
I remember this was a real problem when i started using drupal; I often ran into 2-3 Drupal modules doing something similar, that solved my problem, but i had no idea which one to use. Now you can check how many other people are using a particular module, to give you a pretty good idea of what to use yourself. This data is displayed for each module, and it’s also put together on the Project usage overview page.
First 3 modules by the number of installs, are the core, views and cck. That sounds about right. Those are the main 3 reason i chose Drupal over Joomla.
Not that Joomla should be put aside or anything; it’s a great cms, with some of the greatest contributed templates I’ve seen online.