At one time or another, most Drupal developers have worked on a site with one or more Webforms that used a select list that pertained to some other entity on the site (such as a store location, law firm practice area, etc).
One of the coolest features that made it into Drupal 7 without much (if any) fanfare is the Queue API. Drupal's Queue API is a pluggable system that allows you to queue events for later processing.
If you have a content manager that likes to use inline images in the Body field of a Drupal site, you might run into an issue where they want an image inserted at the beginning of the field but don't want it to display in a teaser.
We've all done it. When we needed to find something in a log, we just did a cat or a tail and piped it to grep. Maybe we told grep to also show us X lines before and after what it found.
Every Drupal developer knows that there many configuration settings that are similar between the sites that they build, whether it's a personal preference or a trend among clients.