If you sell software or services online you need to know your site or web application is actually online!
ServerMojo is a distributed, remote monitoring service which will let you know if your site ever goes down by sending you an alert via email, sms, rss, jabber or even twitter. Handy charts show how your site is performing over time, with uptime/downtime and response times. There are free and premium (with free trial) accounts to cater to all levels of user.
There is also an API which allows you to fully integrate server monitoring within your own applications - you can create and manage monitors and sub-accounts, get monitor stats and more.