Apache JMeter is a subproject of the Apache Jakarta Project that can be used to measure performance on static and dynamic resources such as files, Servlets, Web Services and many more. Moreover, you can also use it as an automated testing environment, as you can easily compare actual results with expected ones using assertions.
Using JMeter is really straightforward and the manual is quite good; just remember to copy mail.jar and activation.jar in the lib dir (as an alternative you can always set the user.classpath property in jmeter.properties) if you want to test Web Services.
The picture shows a graphical example, but it is also possible to view the results in summarized reports, aggregated reports and so on.
We used it to simulate a heavy load on an i5 exposing a Web Service realized as a proof of concept, and drew a comparison with a similar service realized some years ago. We were quite satisfied with the results, as the average response time under stressing conditions has dropped by about 40%.