Site Uptime Monitoring
Web site availability or uptime is especially important if you're running a small business on the web. It is better to automatically learn when there is significant server downtime so you can have it fixed or moved immediately rather than find out from irate customers or a large dip in daily revenue reports.
The easiest and usually ideal solution is to sign up with an uptime monitoring service, free or otherwise, they have servers for monitoring from multiple locations around the world to prevent false alarms in addition to many options to alert you such as: email, SMS, instant messenger, and some even offer phone calls.
Scripts and other software are also available which can be installed on your home computer(less reliable depending on your ISP or computer stability) or other servers you run(can be more complicated to install and may have administrator access requirements), nothing is stopping you from using any and all means to monitor your web site so you can use a combination of local and 3rd party methods.
List of services available to monitor your web site:
Alertra Web Site Monitoring - One of the most popular monitoring services available with many alert and monitoring options and detailed reports.
WebSitePulse - another good service with many options and reports.
Siteuptime - currently doesn't offer page checking and has fairly simple reports but reliable.