Pingdom is a great external tool for monitoring your application uptime. Pingdom can send you phone and email alerts if your site goes down. Pingdom also provides uptime and response time reports.
This page describes how to set up Pingdom to monitor your application.
What pages should I monitor with Pingdom?
In general, do not monitor your home page. This is because your home page may have external dependencies that make page loading slow and thus not accurately reflect your site response time. Moreover, if one of these dependencies goes down, it brings down your home page even when the rest of your application is still functioning correctly. Also, pinging your homepage creates more load on an already heavily trafficked page.
Better practice is to ping a less commonly visited page.