Tsung is an open-source multi-protocol distributed load testing tool
It can be used to stress HTTP, WebDAV, SOAP, PostgreSQL, MySQL, LDAP and Jabber/XMPP servers. Tsung (formerly known as IDX-Tsunami) is a free software released under the GPLv2 license.
The purpose of Tsung is to simulate users in order to test the scalability and performance of IP based client/server applications. You can use it to do load and stress testing of your servers. Many protocols have been implemented and tested, and it can be easily extended. WebDAV, LDAP and MySQL support have been added recently (experimental).
It can be distributed on several client machines and is able to simulate hundreds of thousands of virtual users concurrently (or even millions if you have enough hardware …).