Pingometer
elmah.io
ContentSquare
Atatus
CloudMonix
Visualobserver
Stillio
Sucuri Website Security
Eggplant
Nagios Core
WebGazer
PagerDuty
Freshping (Formerly Insping)
DownNotifier.com
New Relic
Lumio
CloudRadar Monitoring
WebTitan
CloudQA
Uptime