There is similar question to this one here but I guess that by this one I'll get some results and provide added value.
As a SDK developer/provider, I am guessing what to write in the documentation and what the documentation for the SDK should look like. Unavoidable sections that come to mind are:
So please, in you answer, try to provide at least 3 links to a documentation that is 'great' from your perspective, and give some information on why.
If you want to be language specific, I'm targeting C# developers, and my SDK consists of a small set of classes - so the documentation in answers should be similar. MSDN, DevExpress, Ogre - just an examples of something I don't want to be put as an example, because there are many man-days in them, and are great nevertheless.
Using Java and biometrics as an example, standard API docs built using javadoc, like Griaule's at http://www.griaulebiometrics.com/javadoc/FingerprintSDKJava/com/griaule/grfingerjava/GrFingerJava.html, have all the info a developer needs; whereas DigitalPersona has a PDF file with highly-edited API descriptions that don't have the links from method to class to field that a Java developer expects.
Languages and paradigms aside, the more open you can be with your documentation; not trying to hide anything, and keeping your documentation in sync with the actual product, will endear you to your customer base.
More specifically, in your proposed answers to the question, the whole class reference is most important. After that, one sample application that makes use of most of the API, complete with source.