JMeter

If you do performance testing on a regular basis, and you've not used JMeter yet, you need to give it a try. It's free, open source, and very lightweight. I find it's well suited to many of the day-to-day performance test projects, and because it's lightweight I can make new tests and changes to tests quickly.

From the website:

Apache JMeter is open source software, 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.

Apache JMeter may be used to test performance both on static and dynamic resources (files, Servlets, Perl scripts, Java Objects, Data Bases and Queries, FTP Servers and more). It can be used to simulate a heavy load on a server, network or object to test its strength or to analyze overall performance under different load types. You can use it to make a graphical analysis of performance or to test your server/script/object behavior under heavy concurrent load.