Making APIs useable

Shouldn't need to read docs (principle of least surprise)
Explorable API
Click on a link and see results
Realistic Data - facebook
Personalization of API results data - facebook
Easy oauth token generation (without the flow) - twitter
Interactive SDK - bing
Cut & Paste
Document all parameters
Have 1 easy thing - 1st thing
Always have a publci endpoint
API metrics based on my API key (e.g. Apigee)
Web page to API mapping tool - how to get this data via the API
Client libraries
examples & tutorials
Links to other people's libraries
This is a bug button - be able to submit a bug report from the web interactive api that includes request/response/etc
Show base string in detailed error messages
Atom's extensibility & indirection complicates things
Twurl and OAcurl style tools in client libraries
Samples
Social proof (e.g. Apple's mtv crib alike showcase of iphone millionaires)
Behave like an open source project (committers, lists, IRC, code, feedback)
Status pages, e.g. status.twitter.com
Use familiar terminology
Clear versioning policy
Use cases & personas for APIs
Optimize API for common patterns - batching and filtering

Exemplars