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.

corischlegel

I like the very simple API to FirePHP and find it very useful for debugging WordPress plugins and themes. There are a few other plugins out there that integrate FirePHP, notable WP-FirePHP and WP-Logger, but both add complexity that I don’t want to have to deal with for a quick debugging task.
It wouldn’t be that hard to install the FirePHP core and add the includes to wp-config.php, and that’s what I generally do on sites that I control, but having this plugin in the Wrodpress.org/extend directory means that I’ll be able to install it on the fly for a blog that someone hasn’t given me shell or ftp access to.
<h3>Plans</h3>
I don’t think I’ll do a whole lot more with this, since I want to keep it pretty simple, but I plan to make sure the plugin loads first (see wp-logger) and perhaps add a simple options screen. Might also add PHP4 support, but probably not.

Simple WP FirePHP

Extremely simple mechanism for embedding FirePHP into WordPress to use FirePHP during WP Development.

Yet Another Logger Plugin vs Simple WP FirePHP