Different Approaches to an API Ecosystem

I'm reviewing four major Application Programming Interfaces (API) today. Zendesk, Scribd, Twilio, and Evernote.

These are all top web 2.0 or cloud applications. Whichever label you wish to apply.

I am evaluating the different approaches to delivering their API Ecosystem: Twilio represents a new age Software as a Service (SaaS) where the entire product is built around an API offering. So they are kind of a different beast.

I would have to say that Scribd delivers the cleanest API Ecosystem with a great quick start, and robust set of building blocks for their API community. Zendesk and Evernote have great API, but their focus on delivering a complete API Ecosystem is a little fragmented and overshadowed by their core web application offerings.