UptimeRobot!

Pingdom web site monitoring and other infrastructure for you and instantly let you know when anything goes wrong. Measure uptime, downtime, and the performance of your website as experienced by visitors.

PHP Server Monitor is a script that checks whether your websites and servers are up and running. It comes with a web based user interface where you can manage your services and websites, and you can manage users for each server with a mobile number and email address.

Cabot is a free, open-source, self-hosted infrastructure monitoring platform that provides some of the best features of PagerDuty, Server Density, Pingdom and Nagios without their cost and complexity.

Track web page changes and get notified. Free Sign-up. Have all data in one place

The AppBeat Android app lets you connect to your AppBeat uptime monitoring service from anywhere, anytime. You can view current live status of your checks and do basic check management:• Create simple web checks• pause/resume specific check or entire group of checks

WebSite-Watcher detects website updates and highlights all changes in the text. WebSite-Watcher monitors web pages, password protected pages, discussion forums and much more.

RapidSpike is an uptime and performance monitoring service for websites and web applications. It simplifies monitoring, alerting and reporting to support your online presence and to ensure a positive user experience. Organisations use it to identify problems, measure performance and improve capacity planning.

Publisher notes:

SynTraffic provides real-time monitoring for your websites from any geographical region around the world. Smart alerts notify you about important issues as soon as they occur while helping you deal with alert storms gracefully.

FREE online web application that allows you to monitor your server/website: response times, availability, website statistics. Very easy in use even for non-technical people.

Port Monitor is an easy and online tool that does website and server monitoring for you 24/7 every 60 seconds and sends you or your co-workers instant email/push notifications when something goes wrong.

Monitis is a Cloud monitoring and IT analytics service offering Website Monitoring, Server Monitoring, Network Monitoring, Application Monitoring, Cloud Monitoring, and Custom Monitoring.