AsciiDoc viewer for Mac OS X
The Google Chrome browser can render AsciiDoc files.  Just open an AsciiDoc file from Google Chrome using the menu item File -> Open File or the keyboard shortcut Cmd-O, and you should see the "live" file.  

(Edit: I forgot that this requires the Asciidoctor.js Live Preview extension for Chrome).

In your editor, when you make changes and save the file, Chrome will automatically update its view.

Also, some editors (e.g. Atom - see https://atom.io) also have plugins that do live previews.