This is a list of useful resources you can access freely on the web, to learn Go :
Go Playground : Try Go without installing nothing on your computer.
Learn Go with Tests : I like this one because not only you learn the language but also the good practice to test your code.
Go by Example : Showing code example with annotations helps understand common usage of the language.
A Huge Number of Go Examples, Exercises and Quizzes : I think this is a good way to practice a new language, looking for errors in a program and trying to fix them. The format is not easy to follow : it is a git repository and documents are in folders as markdown file or PDF.
This is one of the most famous tribute to Go as a useful language :
How Go helped save HealthCare.gov featuring Paul Smith, CTO of Ad Hoc (Go Time #154 podcast)
This list is a work in progress and will be updated each time I find a valuable resource