OnlineOrNot - Uptime monitoring for software teams (even teams of one).
Pingometer - Pingometer is a service that monitors the uptime, downtime, and performance of websites. Get 24/7 monitoring - sign up for your FREE account today!
Statsignal - Statsignal is a simple and cost-effective way to monitor your website or server and publish a status page of your system.
Uptime Doctor - You can monitor up to 5 URLs with 1-minute monitoring interval, for FREE!
WebmasterSidekick - It is an All-In-One Site Monitoring & SEO Tool that Reports site Performance, Monitoring Key...
AppBeat - AppBeat is reliable website monitoring service that alerts you when there is a downtime or failure. It also measures performance and provides functional tests.