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.
There's a place in Moodle Contrib for people to share their Jmeter scripts here: http://cvs.moodle.org/contrib/tools/jmeter/
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