In the interest of discovering automatically which Drupal modules are the most important to know about, I compiled some statistics. I used a google-like method of quantifying importance: I counted the number of times each module was linked to in the descriptions of other modules (including sandbox projects). The data were gathered yesterday, 11/29/11 from drupal.org. Here is the top 200:
Some links were uncounted, due to limitations of the data set.