I've been learning about Drupal Module Development the past few weeks. I'm using the venerable "Definitive Guide to Drupal 7" and a few times in the module development chapters (18, 19, 20), author Ben Melançon mentions running Drupal "in an IDE" to see how Drupal creates its various internal arrays.
In the world of Java development, you're bound to throw some projects into Eclipse that fill up the Problems View with so many issues that this view becomes unusable. That happened to me, and it quickly reduced my productivity. This view lets you find compilation and build issues that require fixing. The team leader on my project said "you've got a lot of errors there; let's filter them." He showed me how, and I was amazed. Apparently, this great and obvious piece of functionality was available, seemingly right under my nose!