Prometheus - An open-source systems monitoring and alerting toolkit.
Visualping - Visualping is the easiest to use website checker, webpage change monitoring, website change detector and website change alert software of the web. Read more about Visualping.
Datadog - See metrics from all of your apps, tools & services in one place with Datadog's cloud monitoring as a service solution. Try it for free.
Distill Web Monitor - Distill is a web monitoring tool. It can monitor RSS feeds, a webpage or a part of webpage. Alerts in the form of pop-up, audio or emails can be received.
Nagios - Complete monitoring and alerting for servers, switches, applications, and services
Wachete - Track web page changes and get notified. Free Sign-up. Have all data in one place