Apache JMeter is "a 100% pure Java desktop application designed to load test functional behavior and measure performance". Originally designed to test web applications, it is now a general tool for testing client/server applications like database servers, FTP servers and others. It lets you test your applications under different work loads, and graphically represent the application's behaviour. It is also possible to use JMeter for distributed testing (PDF).

Version 2.3 brings quite a few changes, and JMeter 2.2 users are advised to upgrade.

Together with other tools covered earlier, Jmeter should definitely be in your developer's toolbox.