1. Google Developers: Analyze and optimize your website with PageSpeed tools to implement the web performance best practices
2. WEBPAGETEST: WebPagetest is a tool that was originally developed by AOL.The online version at www.webpagetest.org is run by the WPO Foundation for the benefit of the performance community with several companies and individuals providing the testing infrastructure around the globe.
3. GTMetrix: GTmetrix can help you develop a faster, more efficient, and all-around improved website experience for your users.
4. PingDom: Pingdom offers cost-effective and reliable server, network and website monitoring. The service includes statistics for uptime and response time, and can send out alerts via SMS, email, and more.
5. LoadImpact: http://loadimpact.com