Pathologic, a Drupal module which uses some simple heuristics to find and correct broken links and image paths in content which might happen because the content was migrated between servers, the URL of the server it was hosted on changed (such as when they started using HTTPS - early versions of this module predated the "all sites must use HTTPS" idea), the content used relative paths but was being viewed in a context where a full URL was necessary like a feed reader, etc.

It came about when I was working on the site for a local newspaper where the writers would work on articles using an internal copy of the site which would then be migrated to the live site when published, but when they linked to other articles, they would often copy-paste the URL of the article as it was on that internal site, causing a broken link when the article was published. Rather than try to teach a bunch of crusty journalists what relative paths were, I just wrote a bit of code to fix those URLs for them. With the client's permission I later cleaned up and open-sourced that module.

At its peak it was installed on over 70,000 Drupal installations that we know of (not all of them reported usage data). It's still being developed, though by other developers in the last few years as I have lost interest (and client work) in Drupal, but I'm still quite proud of it, and I still love to hear from people who say it was a must-have for their sites and made their lives so much easier.

https://drupal.org/project/pathologic