IsDown App - Monitor all your cloud services status pages in one place
Ctatus - CTATUS helps you monitor your external dependencies, catch problems before your clients, and get leverage over your competitors.
DownDetector - A simple service to tell you whether a website is down.
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.
Instatus - Quick and beautiful status pages.
StatusCake - Website Uptime Monitoring & Alerts – Free Unlimited Downtime Monitoring