I think Ruby has some pretty nice documentation. at ruby-doc.org I love being able to toggle source view for functions and stuff to see how its implemented. I know this is probably violating some "best practice" of don't show implementation, but honestly, seeing an implementation makes it a lot easier to understand what a function does than just having your params and their types listed out...

Two of my favorites:

- Django (perhaps with the exception of class-based views, which are getting better) - https://docs.djangoproject.com/en/dev/

- Stripe - https://stripe.com/docs

I second Stripe. When logged in, they even place your api key into the reference examples, so that you can copy paste working code.

Massively impressed with Parses documentation https://parse.com/docs/

YUI, before the recent version, had some pretty serious documentation and the depth of their examples was spectacular.