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 works great with almost all monitoring tools including: Nagios (and Icinga), Keynote, New Relic, Pingdom, Cloudkick, Circonus, Red Gate SQL Monitor, Server Density, Zenoss, Monit, Munin and many others. 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. (Is my country included?)
In my experience, PagerDuty is the *only* way to get reliable real-time alerts sent to techs with iPhones.

Jesse Newland
CTO,
Rails Machine