If your website is mission-critical for your business, then you need to make sure it's online at all times. The only way to do that is with an uptime monitoring tool. ServiceUptime is our favorite of those tools, and earns our top pick. ServiceUptime has probably the best feature set with the absolute lowest prices around. You can monitor all aspects of your web infrastructure, and can choose the frequency of checks, and be notified of issues via email and/or sms. Check them out!