WordPress is an excellent CMS engine and has lots of plugins that extend the range and functionality of the application.

As a developer, what plugins are most useful to developers.   Any list is open to debate and new tools are allows coming online.  In the most part this list is made up of free tools.

Coming Soon Page & Maintenance Mode
this is a very handy plugin to let people know your site is under construction or redevelopment.

Query Monitor
Accessible from the admin menubar, this tool enables you to see what is happening under the hood.  This has saved me hours of debugging.

Error Log Monitor
This has been brilliant for helping find out what is happening with my PHP code and database calls.

Email Log
The free version doesn’t show a lot of information, but it does confirm if emails from your application are getting sent or not.

Advanced Custom Fields   ( & Pro)
Unless you want to really get into the code, ACF is a fantastic tool to  allow you to create posts with unique data that can be presented in the front end.
The Pro version really builds on the free version.  But the free version is fantastic.

Custom Post Types
Another great tool to allow developers to very quickly modify the basic post type and give you a easy means to categorise groups of data.

There are heaps and heaps of tools, but this simple set of plugins will get you a long way to developing a custom type or application.

My custom Functions
This excellent plugin allows you to add php routines to your theme without having to modify the “functions.php” file.

One added benefit of this plugin is the fact that if you have PHP syntax errors in your code, you will not kill your theme as happens when you make mistakes in the functions.php file.