Libraries/API Wrappers:

This API documents the HTTP calls from GoPro cameras, users have made libraries for several languages for convenience.

Ruby:

konradit/gopro-rb-api - Ruby API for GoPro HERO4/5

NodeJS/NPM:

kschzt/gopro - provides a Node.js API for HERO3 cameras
citolen/goproh4 - Node.js API for HERO4 cameras, available in NPM

Python:

Konradit/gopro-py-api - Python library for GoPro cameras (maintained and updated by me)
joshvillbrandt/goprohero - Python library for GoPro H3/H3+/H4 cameras
DenisCarriere/gopro - Python library for HERO4/5 cameras

Java:

topiltzin-butron/gopro - Java API for GoPro HERO3+

C Sharp:

r1pper/GoPro.Hero - C# Library for HERO2/HERO3/HERO4

GoLang:

chrisjoyce911/goprowifi - GoLang API for GoPro HERO5 (work in progress)

C/C++/Arduino:

aster94/GoProControl - Arduino library for GoPro cameras

Rust:

fschaupp/goprowifi-rust - Rust-API for GoPro HERO5 (work in progress)