FREE UNLIMITED Uptime Monitoring with Google Docs (Every 5 Minutes)
Especially if you're a Hostgator customer these days.
There are free 5 minute monitoring resources like Free Website Uptime Monitoring - Uptime Robot and Free website monitoring service: Montastic, but I've found their reliability a little hit and miss.
Google, however, is pretty reliable.
Here's the step by step instructions for setting up 5 minute monitoring for UNLIMITED SITES with Google Docs
Free Website Monitoring Tool with Unlimited Text and Email Alerts