Using Apache JMeter
Full article can be found here : http://jmeter.apache.org/usermanual/build-web-test-plan.html
Wapt Pro
30 days trial available. 20 users simulation in the trial version. Website : http://www.loadtestingtool.com/download.shtml
LoadImpact Website
Free online testing. 50 users simulation. Website : http://loadimpact.com/