OpenSTA is an open source Web performance tester that runs on Windows.
Here's what Jim Rapoza said about it in an eWeek article :
This product isn't among the best we've seen in any area—it doesn't have the best interface for configuring tests, and its built-in reports options are fairly weak, for example.
But OpenSTA has one clear advantage over its expensive commercial competitors: It's a free, open-source product that, despite its limitations, lets businesses carry out high-level tests of enterprise Web applications.
You can download it at portal.opensta.org