PUBLIC MARKS from amyrussell with tags json & "api key"

I'm sure not everyone is developing Ajax-based front end sites and applications. There must be someone who is developing XML, JSON and JSONP web services, right? So, if you are going to offer it as a commercial service, and even if you aren't, you still want to be able to control and measure access to it. Something like those Google and Yahoo service API keys, right? I'm sure we've all worked with MD5 and SHA-1 enough to have some ideas about how that would be done, but it helps to have an example to follow.