Scene Builder is now open source, as a part of the OpenJFX project

Scene Builder is now open source, as a part of the OpenJFX project .

The whole Scene Builder functionality is provided, including the SB Kit API (designed for integration of SB features into third party tools) as well as the standalone SB application.
This code is made available under the terms of a BSD-style license, similar to JavaFX samples.
The only part of the product which remains closed is the native packaging/installer code.

Scene Builder source code is located in apps/scenebuilder/ in the OpenJFX repository .
Building and running the Scene Builder application from these sources can be done either of the following ways:
From NetBeans: open the SceneBuilderApp project and run it (you need a JDK 1.8 Java Platform configured)
From the command line: 
$ cd SceneBuilderApp
$ ant -Dplatforms.JDK_1.8.home=<path_to_JDK_home> run

source: https://forums.oracle.com/message/11290619

Building OpenJFX on Mac with JDK

Building OpenJFX on Mac with JDK b78
Step by step instructions (with some gotcha's!) in building OpenJFX on Mac OS X with JDK b78