A few years ago I started using Clickatell as a way to send SMS messages from different applications I built. Getting a message to notify you of particular events (“you received an order”, “something isn’t working right”) allows you to respond in a timely manner.
I have one web-based application that monitors various websites I work on and I get a downtime notification via SMS any time a site is down. I have also integrated SMS with osCommerce to send order notifications including the contents of the order. This is great for monitoring a low volume sideline store. There are lots of applications for this of course.
I have used Aleksandar Markovic’s Clickatell SMS API for a number of years. It supports CURL and fopen for the HTTP request and is very easy to integrate into your application.