Pagerduty

A proprietary, but API-accessible alerting service, free up to 5 users.
Alerters exist for ios, android, chrome-extensions, etc.

Documentation

home	https://www.pagerduty.com/
github go-pdagent (cli tools)	https://github.com/PagerDuty/go-pdagent
github go-pagerduty (api client)	https://github.com/PagerDuty/go-pagerduty
google-chrome notifier	https://github.com/PagerDuty/chrome-notifier
API examples, python	https://github.com/PagerDuty/API_Python_Examples