Apache JMeter is an open source Java application, designed to load test functional behavior and measure performance. Originally designed for testing Web Applications, it has since expanded to other test functions as well.
wrk is a modern HTTP benchmarking tool capable of generating significant load when run on a single multi-core CPU. It combines a multithreaded design with scalable event notification systems such as epoll and kqueue.
Tsung (formerly IDX-Tsunami) is a distributed load testing tool. It is protocol-independent and can currently be used to stress HTTP, WebDAV, SOAP, PostgreSQL, MySQL, LDAP and Jabber/XMPP servers. It is distributed under the GNU General Public License version 2.
OctoPerf is a SaaS load testing solution, based on JMeter.
Stress test your web and mobile apps within minutes. Design realistic virtual users. Scales to thousands of user from multiple locations simultaneously. We take care of the infrastructure.