Important WordPress plugins for programmers:

P3 (Plugin Performance Profiler) – See which plugins are slowing down your site. This plugin creates a performance report for your site.

WP safe updates – Test WordPress plugin updates safely before applying them on the live site. Create sandbox where the user can safely test updating plugins without affecting the live site.

General Important WordPress plugins:

Wordfence – one of the best security plugin / toolset for WP, include website firewall.

Sucuri Security – Auditing, Malware Scanner and Security Hardening. Another security plugin / toolset for WP.

Ceceppa Multilingua – Simple & free multi-language plugin for WP.

Accessible Poetry – Add basic accessibility options in your website.

WordPress codex

Make your WordPress plugin talk AJAX

Adding Ajax to Your WordPress Plugin

Using AJAX In WordPress

Example: Building a WordPress Plugin to Create an AJAX Contact Form

Interesting problem:

Cannot login to the website, except the first user.

The file "wp-login.php" was re-uploaded.

All the techniques in WP help didn't work.

In the end – the problem was DB definition: WordPress log-in redirect loop