The JPL Server Monitor is a semi-sophisticated monitoring system. It has the ability to monitor both web sites and server services, like mysql, ssh, telnet and smtp. It currently supports the sending of email and SMS notifications.
JPL Server Monitor (JPLSM) currently supports 5 SMS gateways, though you can add in support for a different SMS gateway with little effort. The notifications can be sent out based on various events, site/service failure, always or only when a site/service status changes.