One of the really great tools we use here at salonMonster is pingdom. It’s a remotely hosted web-service which monitors your website or web applications. They notify by email, SMS text-message almost instantly in case of any issues, incredibly helpful in maintaining a high availability web service. They even have an iPhone app, which shows the status of all services, and your number of “nines” of uptime. Basic free version is available too, just limited to one monitored URL.