MakeUseOf.com:
Mon.itor.us is a comprehensive online site monitoring and tracking service which not only warns you when your website is down but also provides statistics of your website’s uptime, downtime, visitor traffic and more.
It provides a lot of information about your site and web server that helps you maintain a high uptime.
Mon.itor.us monitors your website from multiple locations and sends downtime alerts via email, text message and RSS.
Besides these, it has internal monitoring features like keeping an eye on computer/server memory, CPU, etc. with an installable agent. It also features real-time visitor monitoring.
Photo by Mon.itor.us.