Use Advanced Custom Fields PRO Repeater field in an options page to make a list of information easy for a client to maintain for easy rendering in multiple formats.
Learn how to disable plugins except for specified urls. Inspired by Plugin Organizer plugin, I disabled CF7 for all but the contact page.
If you are running WordPress Multisite, this code will list the plugins active on each of the blogs.
Before discovering the Query Monitor plugin I wrote a plugin to display post/page/server info in a comment. It was very useful when developing WordPress themes.
Debug WooCommerce without admin knowing (not in a bad way) – enable a payment gateway and redirect emails in code.
Automatically disable related videos being shown at the end of the embedded YouTube video.