Open Source load testing tool not known

I discovered setools I was not aware of, so may be helpful for you:

1. Pylot is an open source tool which runs HTTP load tests for testing performance and scalability of web services. It generates concurrent load (HTTP Requests), verifies server responses, and produces reports with metrics. Tests suites are executed and monitored from a GUI or shell/console.
2. Tsung is an open-source multi-protocol distributed load testing tool. It can be used to stress almost any kind of server with HTTP, WebDAV, SOAP, PostgreSQL, MySQL, LDAP, Jabber/XMPP servers. HTTP reports are generated during the tests.
3. Siege is an http regression testing and benchmarking utility. It was designed to let web developers measure the performance of their code under duress, to see how it will stand up to load on the internet. It supports basic authentication, cookies, HTTP and HTTPS protocols. Siege was written on GNU/Linux and has been successfully ported to AIX, BSD, HP-UX and Solaris.

Of course you can also use the most know ones: Apache JMeter, The Grinder.

For microsoft, as usual, their provide some very useful tools:

Microsoft offers 2 tools for stress testing IIS servers:

• IIS 6 Web Capacity Analysis Tool (The IIS 6.0 Resource Kit Tools include WCAT)
• IIS 7.0 Web Capacity Analysis tool
• Web Application Stress Tool

Finally, Browsermob created by Patrick Lightbody (avid open source contributor, having founded OpenQA, created Selenium Remote Control, and co-created Struts 2) is not an open source tool, but offers load testing in the cloud at a very affordable price (per as you go).