PagerDuty - Cloud based monitoring service
InformaCast - InformaCast is a leading mass notification software solution that helps organizations reach everyone with critical information.
One Call Now - One Call Now's message notification delivers automated phone calls, SMS text messages and emails within moments to any group.
DialMyCalls - The easy to use mass and emergency notification platform. Send a phone call, text message or e-mail to your entire list of people in seconds.
OpsGenie - Alerting and On-Call Management for Dev&Ops Teams
Call-Em-All - Call or send messages to friends and family members as well as other people who are online. The app allows you to create a message and send it out to multiple devices and pages.