Is it Down? - Is it down can check if a website is down or is it just your PC that can not access it.

StatusGator - Monitor all your hosted applications, vendors, and websites, and keep your team informed of outages. StatusGator is the world's premier status aggregator, status page, and website monitoring tool.

Is It Down Right Now? - Network & Admin

StatusPage.io - StatusPage.io is the best way for web infrastructure, developer API, and SaaS companies to get set up with their very own status page in minutes. Integrate public metrics and allow your customers to subscribe to be updated automatically.

DownDetector - A simple service to tell you whether a website is down.

IsDown App - Monitor all your cloud services status pages in one place