I'm completely hooked on Google Analytics or rather the information it gives me about how our web site is being used. One of my favourite information sets is the referring web sites. Where did people click through to our web site from.
This throws up some occasional gems where real people are developing against our API and sharing it with their peers.
One that cropped up recently was a plug-in for Nagios an Open Source host, service and network monitoring program. One of the users as developed a Nagios plug-in: SMS HTTP Gateway - Esendex that sends system alerts using our SMS API.
The power of Internet service delivery is demonstrated time and time again by these and similar applications. Nice to be a part of it.