[koenigdavidmj]

Nagios does that too. You can define schedules to your heart's content, and it will send email anywhere. Given that basically all pagers/mobiles have email addresses, you're set.

The only thing that this does that is really new is two-way SMS.

[agmiklas]

PagerDuty also includes voice calls, which we've seen through experience are more reliably delivered than SMS messages (esp. SMSes through email-to-SMS gateways). As with the SMS messages, you can immediately acknowledge or escalate during the phone call using touch-tone.

I also think PagerDuty's ability to graphically define the on-call schedule and escalation rules is much nicer than mucking around with Nagios's configuration files, but I'm a bit biased :)