When your systems go down, PagerDuty will wake you up. You choose how you want to be alerted - via phone, SMS or email, to multiple numbers, with retries.
PagerDuty seamlessly integrates with nearly every monitoring tool, including Nagios, Zenoss, Monit, and Munin. If it can send email, it will work with PagerDuty.
Easily set up schedules to fairly share on-call duty responsibilities with your team.
If you're paged but don't respond in time, the alert is auto-escalated to a team member. Ensures nothing slips through the cracks - ever.
PagerDuty's infrastructure is fully replicated in multiple data centers, with fast failover when problems occur.
Phone alerts can be delivered to over 170 countries and territories; SMS alerts are available virtually world-wide.
In my experience, PagerDuty is the *only* way to get reliable real-time alerts sent to techs with iPhones.

Jesse Newland
CTO, Rails Machine