By 2014, the Rules module is at its peak of usage with over 200,000 reported installations and has gone through 3 major iterations: In 2007, Wolfgang Ziegler, a.k.a. fago created the initial version called Workflow-NG for Drupal 5. The Rules module first appeared for Drupal 6 and was entirely rewritten and enhanced for Drupal 7 as part of his master thesis in computer science. Working on the Rules module also shaped the Entity API contrib module, including fundamental Drupal 7 developer experience helpers like the EntityMetadataWrapper which is used by Search API and many other custom modules.
The Rules module has been featured in many places, including a large book and a small book, on-site DrupalCon trainings and online learning series, screencasts and even a famous Drupal event presentation by Amitaibu.