Example custom Magento profiler via Observer. Please keep in mind this is OLD CODE ;)

config.xml

<?xml version="1.0" encoding="UTF-8"?>
<!-- app/code/local/Example/Profiler/etc/config.xml -->
<config>
        <global>
                <models>
                        <!-- Declare the "example_profiler" class-group -->
                        <example_profiler>
                                <class>Example_Profiler_Model</class>
                        </example_profiler>
                </models>
        </global>
        <frontend>
                <events>
                        <!-- Register the observer for the profiling -->
                        <controller_front_send_response_after>
                                <observers>
                                        <example_profiler>
                                                <type>model</type>
                                                <class>example_profiler/observer</class>
                                                <method>controllerFrontSendResponseAfter</method>
                                        </example_profiler>
                                </observers>
                        </controller_front_send_response_after>
                </events>
        </frontend>
</config>

Observer.php

<?php

// app/code/local/Example/Profiler/Model/Observer.php

class Example_Profiler_Model_Observer
{
        /**
         * Log the profiler statistics plus some information that helps identify the request
         *
         * @param Varien_Event_Observer $observer
         * @return void
         */
        public function controllerFrontSendResponseAfter(Varien_Event_Observer $observer)
        {
                // Only write to log if the profiler is enabled
                $timers = $this->_getSortedTimers();
                if ($timers)
                {
                        $data = $this->_getTableHeadData();
                        $data = array_merge($data, $timers);

                        $out = $this->_getDataAsTextTable($data);
                        $f = fopen('var/log/profiler.log', 'a');
                        fwrite($f, $out);
                        fclose($f);
                }
        }

        /**
         * Return the table head data in an array
         *
         * @return array
         */
        protected function _getTableHeadData()
        {
                $data = array();
                $data[] = array();
                $data[] = array('Request URI: ' . Mage::app()->getRequest()->getServer('REQUEST_URI'));
                $data[] = array('Request Date: ' . date('Y-m-d H:i:s'));
                $data[] = array('Memory usage: real: ' . memory_get_usage(true) . ', emalloc: ' . memory_get_usage());
                $data[] = array();
                $data[] = array('Code Profiler', 'Time', 'Count', 'Emalloc', 'RealMem');

                return $data;
        }
        
        /**
         * Sort by time and return the Varien_Profiler timers array
         *
         * @return array
         */
        protected function _getSortedTimers()
        {
                $timers = array();

                foreach (Varien_Profiler::getTimers() as $name => $timer)
                {
                        $sum = number_format(Varien_Profiler::fetch($name, 'sum'), 4);
                        $count = Varien_Profiler::fetch($name, 'count');
                        $emalloc = Varien_Profiler::fetch($name, 'emalloc');
                        $realmem = Varien_Profiler::fetch($name, 'realmem');

                        // Filter out entries of little relevance
                        if ($sum < .0010 && $count < 10 && $emalloc < 10000)
                        {
                                continue;
                        }

                        $row = array($name, number_format($sum, 4), $count, number_format($emalloc), number_format($realmem));
                        $timers[] = $row;
                }
                usort($timers, array($this, '_sortTimers'));
                return $timers;
        }

        /**
         * Method to compare timer aray entries by timer sum to sort descending
         *
         * @param array $a
         * @param array $b
         * @return int
         */
        protected function _sortTimers(array $a, array $b)
        {
                if ($a[1] === $b[1])
                {
                        return 0;
                }
                return $a[1] < $b[1] ? 1 : -1;
        }

        /**
         * Return the passed in timers data array as a text table
         *
         * @param array $data
         * @return string
         */
        protected function _getDataAsTextTable(array $data)
        {
                // Dummy widths
                $table = new Zend_Text_Table(array('columnWidths' => array(1)));
                $widths = array();
                foreach ($data as $rowData)
                {
                        $row = new Zend_Text_Table_Row();
                        foreach ($rowData as $idx => $cell)
                        {
                                $width = mb_strlen($cell);
                                if (!isset($widths[$idx]) || $widths[$idx] < $width)
                                {
                                        $widths[$idx] = $width;
                                }
                                $row->appendColumn(new Zend_Text_Table_Column(strval($cell)));
                        }
                        $table->appendRow($row);
                }
                $table->setColumnWidths($widths);

                return $table->render();
        }
}