Screaming Penguin - Source Does Matter

JMeter is a completely fantastic testing tool from the Apache Software Foundation (yes ANOTHER great thing from the ASF). JMeter was originally designed to test web apps but has been expanded to test all kinds of stuff. And, note, although JMeter itself is Java based it can of course test any app, not just java web apps.

Check out JMeter here.

For more check this PHPBuilder intro tutorial: Regression Testing With JMeter.