JMeter
From MoodleDocs
Apache JMeter is a 100% pure Java desktop application designed to load test functional behavior and measure performance. It was originally designed for testing Web Applications but has since expanded to other test functions.
For more information about JMeter and what it could help you to stress test your Moodle installation, please see: http://jakarta.apache.org/jmeter/
For JMeter FAQ, see: http://wiki.apache.org/jakarta-jmeter/JMeterFAQ