http://runslikeclockwork.com/
Very simple web monitoring system that checks if a site is online and sends alerts by SMS If down. They also offer a free monit aggregation service. Plans start £3/month basic, £5/month for 5 sites every 30 seconds plus 15 sms credits / month.
http://www.pagerduty.com/
Takes alerts from monitoring systems and sends emails, phone calls and SMS messages. Handles on-call schedules, escalation, acknowledgement, etc.
