Comprehensive monitoring solution with over 200 sensor types, user-friendly interface, and powerful capabilities such as traffic analysis, failover clustering, and remote probes. Offers email and SMS alerts, supports SNMP and WMI, provides high-availability features and detailed reporting.

StatusCake.com is a free website monitoring tool which allows users to track up to 10 websites and get alerted when their site is down or is having trouble. Downtime root cause analysis is included in the free plan to help users diagnose and fix issues.

Uptime.com: Real-time availability monitoring & response for preventing website downtime. Your complete website monitoring solution.

Enterprise-ready monitoring solution that provides unparalleled flexibility for IT to address both immediate and unforeseen operational issues, including infrastructure and IT processes.

NodePing Server Monitoring and Web Site Monitoring. Simple but powerful Internet service monitoring. Checks as frequent as once a minute.

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

Dead Simple Uptime Monitoring.

Pingbreak is an original and free monitoring service. Get notified through Twitter if your website is down. Pingbreak checks all of your websites each minute and alert you instantly by Direct Message (DM)

Sematext Monitoring is a full-stack IT infrastructure monitoring software that gives you real-time visibility across your infrastructure. Whether on-premise, in the cloud, or a mix of anything in between, Sematext can monitor your whole stack.

Silent Down is a website uptime and performance monitoring platform, to keep an eye on your application and instantly alert you if any issues arrive.

Monitive is an uptime monitoring service, where users sign up and input their website address, which we check every minute, from a random location around the world, and instantly notify them when their site is down.

Proactively monitor your websites and online services. Receive immediate notification when a problem is detected so you can quickly resolve the root cause and prevent potential escalation.