AlarmPoint Systems is a competitor, but they sell enterprise software, with the usual enterprise tagline of "call us for pricing and a demo" and 5 digit+ price tag. Another one is ReliableResponse. This is a smaller company, but also enterprise software.
We have tried to offer a SaaS solution aimed at small and medium businesses.
I'm the owner of Reliable Response. As alexsolo said, we're an enterprise product, meaning we install on the customer's servers, behind the firewall. Pagerduty looks like a great product for smaller teams or companies which prefer a SaaS solution. Our price point is around $10k, plus yearly maintenance and support fees, so it's definitely geared towards bigger teams.
We have tried to offer a SaaS solution aimed at small and medium businesses.