What are some alternatives?
When comparing Site Down and Montastic, you can also consider the following products
Ruler - It's a screen ruler for Windows.
UptimeRobot - Free Website Uptime Monitoring
IsItDown RightNow - "Is It Down Right Now" monitors the status of your favorite web sites and checks whether they are down or not.
Pingdom - With website monitoring from Pingdom you will be the first to know when your website is down. No installation required. 30-day free trial.
Site24x7 - Site24x7 offers both free & paid website monitoring services. Monitor websites remotely and receive instant email/sms alerts if your website becomes unavailable. View uptime & performance graphs of your website monitors.
Are My Sites Up - SIMPLE Website monitoring. When your websites go down, be the first to know.