P3 (Plugin Performance Profiler)

By GoDaddy.com

See which plugins are slowing down your site. This plugin creates a performance report for your site.

Categories: General

Debug Bar

By wordpressdotorg

Adds a debug menu to the admin bar that shows query, cache, and other helpful debugging information.

Categories: General

Query Monitor – The developer tools panel for WordPress

By John Blackbourn

Query Monitor is the developer tools panel for WordPress.

Categories: General

Debug Bar Console

By koopersmith

Adds a PHP/MySQL console to the debug bar. Requires the debug bar plugin.

Categories: General

Log Deprecated Notices

By Andrew Nacin

Logs the usage of deprecated files, functions, and function arguments, and identifies where the deprecated functionality is being used.

Categories: General