The reference pages for Mathematica are by far the best docs I've seen for a programming language. They're thorough and each example can be evaluated in place.
Online version here [1] is not evaluatable and is rendered to images, but otherwise it's the same content as the desktop version. Specific example [2].
[1] http://reference.wolfram.com/language/ [2] http://reference.wolfram.com/language/ref/Histogram.html
I like the official git documentation. There are many tutorials out there but reading through the official doc always makes things clearer to me at least
I also like Stripe documentation
ZeroMQ Guide has frequently been mentioned on HN as one of the better pieces of software documentation:
It is useful not just for using ZeroMQ, but about learning about distributed systems in general.
https://www.digifire.in/2016/02/06/interview-with-apoorv-aro...
It's a recently written tech interview for a startup named Baatna. Good read!
https://freebsd.org/handbook
It's a comprehensive, yet not too dense, set of documentation for getting people up to speed on how to do things with FreeBSD. You may not find a better one even today.