There are many great modules that are not a part of the Drupal Core. Yet. Here are a few. Some will have more detail than others; descriptions may come later.
CTools – Chaos Tools. Needed by so many other modules that you might as well download, install and enable the suite.
Administration – Excellent module. Use the Tool bar here and disable the one from Core
Views – Have to have it, have to learn it
Module Filter – My new favorite. Gives great summaries for the modules installed and categorizes them in a helpful view