AppKiDo is a browser for the Cocoa documentation that comes with Xcode. There's a more detailed explanation at appkido.com. Here's an old screenshot:
AppKiDo stopped working when Xcode 6 was released. I have resumed work on it, and as of Jun 12, 2016 it is in rough shape, but it runs, and it's able to load Xcode 7 docsets. Presumably I will find out this week whether it works with Xcode 8.
The code should be considered unstable; I'm still changing a lot of stuff.
This intro is out of date but gives a general idea of how the code is organized.