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.

nick000000

This is probably a tool that every wordpress developer needs. Use it to see through the Firebug console all important things like database queries, $_POST, $_GET, $_FILES and other evnironment details.

Developer monitor by VentureGeeks

Check all the running queries and other development information in Firebug. Great to debug performance issues in live web sites.

Yet Another Logger Plugin vs Developer monitor by VentureGeeks