Ask HN: What are some good lightweight server monitoring services?

I'm one-person team and I'm just looking for a service that will ping my servers at some interval and send me an email or text if the response doesn't match some pre-configured match parameters.

Pager-duty would work but 10 bucks a month seems too expensive. I know there must be better alternatives out there.

update: Looks like I misunderstood pager duty. It doesn't handle the uptime checking; it just manages notifications and escalation.