Useful API design best practice

I've been thinking a lot about API.  API is at the intersection of SaaS, consumerization of IT and applications.  As more software moves to services on the cloud, it's becoming increasingly important to interface with the hosted services.  

Analogy that I have in mind is Lego blocks.  Just as Lego blocks can be stacked to create interesting things, APIs can be chained together or nested to create interesting applications.  littleBits is a good example of mixing these two metaphors into actual product.

While researching to get quickly up to speed with the best design practice for REST API, I found the Web API Design PDF written by Brian Mulloy to be very valuable.  Mulloy shares his practical recommendation of how to design Web API.  They are immediately actionable, and makes sense overall.

Practical guide to designing easy to use Web API
by Brian Mulloy can be downloaded from this link.
Image Source: http://www.apibook.com/

Good API should make it easy to develop with API reference guide.  Great API should make the API intuitive enough so that developers can browse the guide once and start using it.