Monitority - Simple website monitoring service (sms/email alerts)
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!
Suprvise - Know instantly when your sites are down.
Happy Apps - Uptime Monitoring Simplified | Happy Apps
PingPing - A simple tool to monitor your website and SSL
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.