Link to PDF of slides http://lcsd05.cs.tamu.edu/slides/keynote.pdf

Google Tech Talks. How to design an API: best practises, concepts, technical aspects.

Here are the main concepts I tried to apply when designing the API:

Easy to learn ; the documentation provides simple examples, complete documentationEasy to use ; single entry point, standard parametersHard to misuse ; explicit error message suggesting parameters valuesAppropriate to audience ; I talked about such a service with several users, and looked at what the competition was offering