Dynatrace Load - Dynatrace Load lets you test the performance of web and mobile applications with the realism of load generation from the cloud.
Pingdom - With website monitoring from Pingdom you will be the first to know when your website is down. No installation required. 30-day free trial.
Apache JMeter - Apache JMeter™.
StatusCake - Website Uptime Monitoring & Alerts – Free Unlimited Downtime Monitoring
Blackfire.io - Blackfire empowers all PHP developers and IT/Ops to continuously verify and improve their app’s performance, throughout its lifecycle, by getting the right information at the right moment.
Better Uptime - We call you when your website goes down