This small web service listens on port 15000 for HTTP GET requests on the /ping path. It's intended to be used with httpmon for Android which should ping that URL every 5 minutes. If the service isn't pinged on schedule, it runs an external script with the argument "offline". Once regular pings return, it runs the script with the argument "online".
I originally wrote it to automatically change how Google Voice text messages are delivered based on whether or not my phone has a network connection. (In many rural areas, phone service is available but data is not).