Tsung - Tsung is an open-source multi-protocol distributed load testing tool.
Loader.io - Loader.io is a simple cloud-based load testing service
gatling - A high performance web server. Small! (125k static Linux-x86 binary with HTTP, FTP and SMB support).
WebLOAD - WebLOAD - The most flexible and cost effective software for enterprise load, stress and performance testing, integrated with DevOps processes. Click for details
locust - An open source load testing tool written in Python.
StresStimulus - Load testing tool for websites and mobile that works with hard-to-test applications.