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 seemlessly 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
We’re pleased to announce that Cloudkick is the first monitoring tool to include out-of-the-box integration capability with PagerDuty.
We’ve just released a Nagios API for PagerDuty.
The new integration API allows you to add PagerDuty’s advanced alerting functionality to any system that can make an API call.