The first two examples that come to mind of what I would deem "good" APIs are: * GitHub: http://developer.github.com http://developer.github.com * Stripe: htt

The first two examples that come to mind of what I would deem "good" APIs are: * GitHub: http://developer.github.com http://developer.github.com * Stripe: https://stripe.com/docs/api https://stripe.com/docs/api Both are easy-to-use and very well documented. The examples listed in Stripe's actually work - they're hooked up to a test account, so you can copy-paste into your terminal and start interacting with it immediately. -- Edit -- To give you an example of a bad API: I'm currently working with a large corp's API, and it's just awful. It's horrendously slow (10+ second response times); everything hits a single URL; the HTTP status of the response is always 200; node names in the response are misspelled and trncated awkwardly; the documentation is virtually non-existant; some response nodes provide inaccurate information. The list goes on.