Velocity

An API,short for Application Programming Interface, is basically a set of routines, protocols, and tools used for building programs. In a few words, APIs specify how certain software components should interact one with each other.
Velocity is a highly useful application that allows you to access over 150 API documentation sets, available from the macOS app, Dash.
Before anything else, you should know that Internet access is required to download the desired API sets and, evidently, once they have been downloaded, you will be able to access them in offline mode.

Install, launch the app and start browsing API docsets in just a few seconds

Subsequent to its typical installation and upon launching Velocity for the first time, you are met by a somewhat barren main window. The utility provides you with a quick way to download new docsets or to manage the already existing ones.
Just choose what docsets are of interest from you, click the designated 'Download' button and start browsing them in no time. Since the app's interface is quite user-friendly, you should get to grips with the app's minimalist set of features is no time.

Has almost all the programming languages covered

Probably the most impressive thing about Velocity is the fact that it provides you with a plethora of API documentation sets.
You can access API docs for .NET Framework, C++, HTML, Mono, Perl, Python, Ruby, SQLite, Wordpress, Arduino, Android, Java SE, Luem MongoDB, Scalat, Unity 3D and you are barely scratching the surface.

Information about API protocols and tools at the tips of your fingers

Another worth-mentioning feature is the fact that the utility enables you to add docsets from the user-contributed repo maintained by Dash or even to add XCode API docsets for iOS and macOS.
You will certainly find it useful that the app also provides you an efficient way to manage the downloaded docsets from the app's main menu section. Just as effective is the utility's search function, that with the help of smart keyword recognition algorithm, offers blitz-quick results.

Novice-accessible and comprehensive development tool

All in all, Velocity is a useful app that offers you access to API docsets for the most popular programming languages out there and a quick way to search for and pinpoint desired protocols and elements.