Zeal is a simple offline API documentation browser inspired by Dash (OS X app), available for Linux and Windows.Discontinued
The last version (0.6.1) is from October 2018.
Dash is an API Documentation Browser and Code Snippet Manager. Dash stores snippets of code and instantly searches offline documentation sets for almost any API you might use. You can even generate your own docsets or request docsets to be included.
Search Golang std docs and third packages in your address bar instantly! Golang Search is a handy browser plugin to help the Golang developers searching standard docs and libraries in the address bar instantly.