shawfactor

This plugin logs all wordpress queries in the current page request as a html comment at the end of your output.
It only does so when the get variable lh_log_queries is set. That variable has three settings. 
1. queries: this logs queries only 
2. hooks: this logs hooks only 
3. both: this logs both hooks and filters
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LH Log Queries and hooks

Decisions not options for query and hook logging

tierrainnovation

Render database profiling at the bottom of all WordPress pages. To install, upload <code>wpdb-profiling</code> to the <code>/wp-content/plugins/</code> directory, activate the plugin, and enable / disable features from the wp-admin plugin screen.

WPDB Profiling

This plugin will give you the total number of queries to the db per page, as well as the total time it took to render those queries out to the page.

LH Log Queries and hooks vs WPDB Profiling