This plugin provides an API to send logging and debugging data via FirePHP or 
via e-mail.
It provides 3 logging functions (wp_yalp_info, wp_yalp_warning, wp_yalp_error) for 3 log levels: error, warning, info. You can call these functions in your code (after plugin initialization) to send your data to the logging system. These functions receive 2 parameters: the first one, mandatory, is the object that you want to log (string, array, etc.) and the second optional plugin is a label to identify your log data.
The log data is automatically sent via the FirePHP system if the current user is the administrator or his IP address is enabled to receive it. If configured, the plugin also sends the log data via e-mail to the website administrator.
This plugin also provides some optional automatic logs for database queries, e-mail 
and posts meta.

Yet Another Logger Plugin

Provides logging and debugging data via e-mail and FirePHP.

Arnaud Lemercier

Use Firebug with FirePHP to analyze the SQL queries made by WordPress. Ideal for development and avoid performance issues.

<h3>Credits</h3>
<ul>
<li><a href="http://arnaud.lemercier.me" title="Développeur Web Rouen" rel="nofollow ugc">Arnaud Lemercier</a></li>
<li><a href="http://wixiweb.fr/" title="Agence développement WordPress Rouen" rel="nofollow ugc">Wixiweb</a></li>
<li><a href="http://firephp.org/" title="Firebug Extension" rel="nofollow ugc">FirePHP</a></li>
</ul>

Wixiweb FirePHP Queries

Use Firebug with FirePHP to analyze the SQL queries made by Wordpress. Ideal for development and avoid performance issues.

Yet Another Logger Plugin vs Wixiweb FirePHP Queries