One of Drupal's greatest assets is the amount of existing contributed modules. Think of a feature and you will probably find one or more modules providing that. Sometimes there are 3 or more modules targeting the same problem. It can be difficult to choose the correct one.
Over the last 14 months I have been working a lot with Drupal, Drupal 6 to be more specific. A long with each new project, we have often stumbled upon certain modules, over and over again. Currently we are using about 10-15 same modules in all projects.