Brett

Frontend Features: 
 – adds a toolbar to the bottom of WordPress pages that will show meaningful information that helps developers to create optimized WordPress sites. 
 – Info includes SQL queries, query vars, peak memory usage, and a filetrace.
Backend Features: 
 – Provides a very basic checklist with suggestions for development. 
 – Shows basic server stats and information 
 – phpInfo 
 – WP rewrite rules (coming soon)

Developer Toolbar

Adds easy to access data for developers, including a fixed toolbar to the bottom of pages that shows query data and memory usage among other data.

asadkn

A simple plugin to assist both theme and plugin developers in MySQL Profiling. Once installed, MySQL statistics will be available at page footer. Visit http://wpsplash.com/mysql-profiler-plugin-for-wordpress/ for more information.

WP MySQL Profiler

Used to profile MySQL queries for performance; with backtrace information for each query (caller functions, lines, file names etc.)

Developer Toolbar vs WP MySQL Profiler