If you are talking about company's internal developer facing documentations, they are not going to be on internet.

Some of the best product documentations there are:

https://stripe.com/docs/api

http://www.twilio.com/docs

https://docs.djangoproject.com

https://developers.google.com/maps/documentation/

http://docs.oracle.com/javase/7/docs/api/

https://parse.com/docs/ios/guide